Intermittent overheating whilst internal heater blows cold!!

[JamJar]

can anybody assist...
I have a 1995 2.5 diesel.Which in last six months has had new radiator, and pump...

In the cold weather it has been making quite a bit of white exhaust when cold, and runs fine a when the engine warms up and exhaust returns to normal.

In recent journeys sometimes the internal blower will only blow cold air, ( even when temperature turned up to 32 degrees) and the engine quickly overheats ( to the point of bubbling in the expansion tank). And next journey the blower works fine, and the engine temp runs fine.
I m no mechanic (in fact I m clueless), but have a good mechanic who has worked on my bongo since I got it 3 years ago. The coolant system seems to have a good few idiosyncrasies, so thought any direction for him to start would be helpful!
I ve isolated the thermostat near to the hand break, to no effect.
Which thermostats drive what and where are they please? Any ideas? ...please...

[haydn callow]

First thing to do is bleed the cooling system correctly...have a look at the YouTube videos (search bleeding bongos) or go to the website below for links......you have air in the system.
I suspect you have overheated and cracked the cylinder head...but... Try the above first.....
If it does need a new head, get it done by someone who has worked on Bongo heads before or you could be wasting your time/money