Hi BBBBB - very sorry to read of your problems. My experience was similar although, as it only happened at low speed, the effects were less dramatic. The head on mine turned out to be fine, but if you do need a new one, get the best quality you can afford. Stay clear of cheap ones advertised on ebay for a couple of hundred pounds. Expect to pay £400+ for a good quality head. It may be cheaper to go for a replacement engine, but then you have no way of knowing whether it is any better than the one you've already got.
There is a wealth of information on the forum from folks who have have been through the same thing. I've had a dig thru some of my posts & put a selection here. Sorry if they don't 'flow' very well, but I've only had time to do a copy & paste job. Good luck.
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This was a summary of what I did:
1) Had head skimmed & pressure tested (cost £90 - mates rates)
2) Re-fitted with new gasket - still had air coming out expansion tank
3) Took head to Bellhill Garage to have it pressure tested again. Their supplier can test it a higher temp which is more likely to cause an cracks to expand. (cost £60)
4) Followed Bellhill's instructions to the letter & 'decked' the block which involves sanding the block mating surface using very fine abrasive paper over a piece of grantite. You will need to remove & replace the oil feed pipe (v. tricky) & 2 locating pegs
5) Used Wellseal on the block & head to ensure no leaks
6) Stuck it all back together & it's still fine 5000 miles later
Total cost of all the bits about £400 including timing belt x2 (never re-fit once it has been stretched), new silicon pipes, clips, head bolts etc. I did all the work myself over several weeks, so no idea of the time involved. If I'd have found out the info from Bellhill earlier, then I wouldn't have had to strip it down & re-fit for a second time.
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CHANGING HEAD GASKET
There's a factsheet in the members area about changing the head gasket which should be enough to get you thru. Don't forget to fit new cambelt & cylinder head bolts. I can't recall exactly which size sockets I needed, but you will definitely need 10, 11, 12, 13 & 15mm. The two 'tricky' ones are deep sockets to remove the injectors (although you may get away with leaving these in place) and something big enough to fit the crankshaft pulley bolt. You need to be able to turn the engine two revolutions as part of the cambelt tensioning process.
I would also recommend getting the head pressure tested whilst its off & then consider having it skimmed. Expect to pay about £60 for each of these, but at least you'll know the head is sound before you got to the expense & effort of refitting it.
Have a good read up of posts on here - most of the answers any queries you have have are buried in here somewhere.
You may find some useful info in these two posts, especially about 'decking the block' & Wellseal:
http://www.igmaynard.co.uk/bongo/forum/ ... =+wellseal
http://www.igmaynard.co.uk/bongo/forum/ ... =+wellseal
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PRESSURE IN THE SYSTEM
I've had a similar experience although I didn't have the overheating part, just the loss of coolant, which would have probably resulted in overheating if left unattended. Unfortunately, overheating has to be the most covered topic on this forum & this is a lot of useful info here, as well as some other posts which are slightly mis-leading.
A good starting point is to use the search facility & also look at the factsheets in the members section.
Your very first thing to check is the state of the coolant. If its brown or just plain water, you should drain & back-flush & refill with a 50/50 mix. If it's brand new, that could indicate the previous owner has already tried (& failed) with this method. The most important thing with anything to do with the cooling system is to following the bleed process methodically. It can take over an hour to do it properly, so following the instructions in the factsheets & on the forum.
Pressure in the system does tend to point to one (or more) of the following:
1) Head gasket failure - where it is no longer performing its job of keeping the water, oil & combustion gases seperate
2) Warped / cracked head - typically caused by an overheating incident
3) Cracked block - this is the cast iron lump that contains the cylinder bores. There is a water jacket aound the cylinders & there is only a few mm of metal to keep it all seperate. I have only come across one Bongo owner who has experienced this, so it is extremely rare.
4) Another owner had a seal fail in the turbo so that coolant was being forced into the engine, but again, this is a very rare occurance.
The symptoms you describe of water coming out the expansion are typical either of an airlock (so the water is not circulating properly & is literally boiling like a kettle), or combined with the white smoke on start up, it will probably be gasket or head failure.
This is a post that covers my experiences:
http://www.igmaynard.co.uk/bongo/forum/ ... n&start=75
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CHANGING THE CAMBELT
It's easier if you have access from underneath as well. There's a really annoying hose from the intercooler that is worth removing to allow better access to the cambelt cover. Also, you need to be able to get a bl**dy big socket (28mm?) on to the crank shaft pulley to turn the engine over & that is best done from underneath. Also worth removing the glow plugs otherwise it will be very hard work to turn it over by hand & get it back to TDC.
Their is a factsheet in the members area which should give you the info you need. If I was doing it again, I could probably do it in about an hour, but thats only because I've already changed mine twice (as a result of cylinder head removal). If you've got all the tools, the instructions & possibly a helper to check its all lined up, then a couple of hours should be enough. Be prepared for bruised knuckles & a fair bit of cursing the idiot that decided to stick the engine in the middle
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WHAT PARTS TO REPLACE
I'm having problems with excess pressure in the coolant system at the moment. Despite bleeding as per the various methods, the problem still occurs, so I plan is to take the head off for a closer look. I will be using the info in the manual & relevant factsheets in the members section. Here's my list of items to get/change whilst doing this. Is there anything else you recommend & any pitfalls to be wary of?
* Headgasket set inc: Head Gasket, Rocker Cover Gasket, Cam Shaft Seals, Rocker Cover Seals, Valve Stem Seals & Manifold gaskets (inlet and exhaust). I won't bother changing valve stem seals
* Cambelt
* Cambelt tensioner spring
* Cambelt tensioner - is this necessary? It has supposedly been replaced before - do they wear out?
* Head bolts
* Thermostat
* Replace main hoses with Flippa's silicon pipes
That little lot comes to a tad under £300 from the club shop, Flippa & Bongobits. Items that I might change:
* Water pump - if the impeller, bearings & seals are ok - is it worth it? What's the typical lifespan?
* Radiator - there are no leaks or obvious hotspots. The water goes in red hot & comes out practically cold. I'll give it another good flush, but I don't believe it will be contributing to current problems. Does anyone know the exact fluid capacity of the rad? I was going to measure how much liquid it holds which would tell me if the capacity was reduced by silt & gunge.
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PS -I've got a new rad on order, so will measure the capacity of the old & new to see if there is any difference. I may also take the old one apart to find out what sort of condition it is in after 10 years.
. Whatever you do I wish you luck. Sorry I dont know anything about cooling etc. It's never been a problem with our Freda.
I'm not as scunnered as I was this morning when I first came to the realisation that this had happened (the PMS could have had something to do with this 
). It had over heated once before when the AA man trimmed a hose and didn't bleed the system like I told him to and told me it would be fine (oh, so naive!!!). I agree it's the poor maintenance of the Bongos which causes the cooling issues. Haydn, I think that if we hadn't got your alarm the Beastie would have failed long before now. So I can't thank you enough for inventing such a marvelous thing! The guy who sold it to us said one of the hoses had gone on him once - in hindsight, if we had known what we know about hoses and cooling I don't think we would have touched it. He never had an alarm so goodness knows what damage may have been done. 
We will see what happens. It's not going to do any harm sitting in the driveway. Going to shop for a little run about tomorrow so we can both get to work. ONE good thing about owning a Bongo - you learn SO much about them, it makes you very clued up when going to look at second hand cars. Oh, hey, I can bleed a system, change a hose, do glow plugs, anti roll bar drop links and an oil change and I couldn't do anything before. That's all thanks to this website! 
, I never usually take that to work because husband always has it. Hope that you get things sorted soon and I'll look out for you towards Erskine Bridge. 