Well of course I completely forgot about this... if anyone is interested, the problem was corroded porous fuel lines! As it was a diesel, with the injection pump at the engine, the fuel line going from tank to engine is always at negative pressure, as the pump in the engine bay is sucking the fuel all the way from the tank. Henceforth, the line would suck air in only whilst the engine was running, but wouldn't leak (noticeably anyway) when turned off. No wonder I never fixed it, I never thought to consider that, but at least now I know for the future.
That car dealer that bought it off me replaced the fuel lines and it ran like a champ again. I know it's had at least two more owners after him, so this car doesn't half move around!
Fingers crossed it's still going strong out there. 
