Update and potential success!! (Tempting fate by saying that)
Booked van into garage for Wednesday but thought I'd have a final bash at locating the problem.
I did find that the vacuum hose to the air filter box (the one that operates the flap to warm the incoming air by passing it over the exhaust) seemed to be working (I could hear the flap actuating) but I could also keep sucking through it - disconnected that and blocked off that vacuum port on the carb and we seem to be back in business!!!
Just back from test drive now and it all seems good ...... will let it all cool down and try again in a few hours just for peace of mind ..... But hopefully that's a success!!
Thanks everyone for all the help and suggestions - it does motivate you to keep searching for a fix when folks are coming up with lots of places to look - so thanks!!

(Alternatively it's all a massive coincidence and the van has temporarily self-healed and this problem will rear its ugly head next time we are 100 miles from home, in the rain, at night ......

)