OK, brief update.
With the (naff secondhand) battery charged AND some jump leads and fresh petrol, I got it started today.
Once started it ran and idled sweet, with no rattles, knocks or other noises.
Left it idling for about 10 mins and was then getting worried about some smoke off the engine, but managed to trace this to an oil leak from a loose allen bolt in the head. Snicked it up with a dab of threadlock and it seems to be cured.
Took it for a quick ride, and it pulls well and rides nice and firm, and the brakes are strong.
All electrics, lights, horns, indicators now work perfectly, and instruments are working as well.
I do have a brand new battery for it which I'm loath to fill with acid until I know what I'm doing with the bike, but the starter motor was very sluggish trying to start it. One of the leads is really naff, so I think I'll try and source a replacement and try to get the thing starting off the button, then I'm really tempted to MOT it.
Come on you lot, SOMEONE must have an idea of value??