Welcome on board David - sorry to hear its not going too well. 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 faclity & 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
If you want to drop me a pm, I'd be happy to give you my number to discuss this further.
