Quick update, in anticipation of the mechanic coming tomorrow I've installed a good battery and had another go at starting the engine. Engine turned over, started and then immediately died as per when we last used it 18 or so months ago. This repeated several times.
I then employed the method that I'd used previously of turning everything off leaving the car for 10 minutes and then starting it with the accelerator fully depressed. As before this worked and the engine started although it was very lumpy for a couple of minutes or so. I kept revving it and eventually it calmed down and ran smoothly.
I wanted to take it out on the road but it's SORN with no tax, MOT or insurance so I had to make do with backwards and forwards on the drive a few times. It seemed quite happy so I got out and cleaned a few bits up for 10 minutes or so while it was idling at about 7-800 revs, very smoothly I might add.
I then turned it off. Returned about 20 minutes later and tried to start it normally and it did start without the accelerator fully depressed but only just and the engine management warning that had reset from having no battery for 18 or so months immediately came back.
One other thing I noticed was that whenever I put an load on the electrical system, turning on the lights, fan etc it would immediately cause a rattling noise from somewhere in the engine bay. No load, no rattle, turn the lights on, short rattle then quite again. Turn the fan on, short rattle then quiet again and so on. Do you think this is indicative of a weak alternator?
Oh and one other thing, I opened up the petrol filler cap and filler tube has gone rusty for some reason, should I worry about flakes of rust coming loose and getting into the petrol when filling up?