Thanks for your replies.
The garage isn't on the list of bongo friendly garages but we have been using them for a while and the head mechanic used to have a bongo. He demonstrated to us that the engine is warped. We're in Edinburgh.
Think it might have to go up for sale!

Hi everyone, just after some advice.
We have sadly been told by the garage that our Bongo engine block is warped due to overheating and it will cost more than the Bongo is worth to fix it.
So I'm wondering what to do with it. It is a 96 N reg with aft and fully converted. It is quite an unusual ...