Oh, hell! I think I'll just cut my losses and sell the van for spare parts to what's called a "scrappy" in the UK and then take a risk on a slightly bigger van (anyone got any opinions on the Vauxhall "Astra"?!). I'm very grateful for all the suggestions and advice, and, in answer to some of the specific points made above:

D el gnomo: the alternator was replaced about two years back, the original one had "frozen-up"; also, it IS the ignition light which isn't going off (the manual states, if this happens, I should drive directly to Ford dealer - but I bought the van second-hand, from a reputable dealer in commercial vehicles)

J MacK: I think a Ford dealer has already told me there isn't any electric fuel cut-out in the Diesel model (but that was two years ago also, and I might have misunderestimated...

Willie-O: yes, I thought the Ford Fieta van does look quite "cute" and even elegant, and it also allows me to use it as a mini-mini-caravan with a sleeping bag and blankets. No, I've not been towing anything (and neither I nor partner are ourselves "gargantuan")

Not using biofuel, and the cap to the fuel-tank is secured. Filters were changed a year back (and ?"fuel lines" had compressed air blown through) Also, had a new timer-belt fitted.

Once I can find someone to help with towing, I'll take it at night (empty roads) to the specialist mentioned by the mechanic I go to for MOT certificates; for those from North American continent, that's a "Ministry of Transport" test to check that certain features are working properly - done on vehicles more than three years old.

Again, many thanks for your interest. I'll revive this thread once I get the problem diagnosed, and/or sorted (or, as I feel like doing, have the van quietly "put to sleep". We've had some good camping trips, sometimes associated with music festivals, together. "sob")