It's been a busy old day but I think I've made good progress with the cutting out / not idling problem, thanks to all the tips I've been given on here. I've had the air filter out and that looked (to my untrained eye!) fairly clean and in good condition (was hopefully replaced when the van was serviced at a local garage last Oct). Took the top off the carburettor and gave it, and the large hose that connects it to the air filter box, a wipe out because there was thick, black liquid in there that looked like old oil to me. Can oil get in here somehow?
Also had the drinking straw out and blew down the jets in the carb again. Coolant has been topped up (AGAIN!

) as well.
After doing that I drove around for 15 mins with no problems!

Then I went and put more petrol in (different garage to previously) and after doing that, started the engine again but then it cut out immediately! Did this a few times so I started it again, kept the revs up and left the garage but for the next 10 mins as I was driving around the engine cut out every time I slowed down. But I just kept driving around and the problem seemed to disappear!!!

It's been great since then (but I haven't been very far in it today).
So putting more petrol in seemed to start the problem off again temporarily which makes me wonder if I'd disturbed some crap in the petrol tank when I put more fuel in?
I'll be keeping my fingers crossed and a close eye on the situation (as well as trying to find and fix the coolant leak) over the next few days. Thanks again to everyone for all the help, and to Big Tam who very kindly gave up his Sunday afternoon in order to wire-up our relay so we've now got intermittent wipers!!!
Would like to try and get a sticker for the tailgate that says "This van is powered by Club80-90.co.uk"
Cheers
Tony