Thanks for the ideas.
Dunno when the ignition leads / cap / rotor arm were replaced, but I gave them a good look over and they appear to be in very good condition - no cracks or burns etc. Spark plugs were done 2 years ago, but have only covered 5000 miles or so. The are a nice coffee colour with no electrode burning, but I might still try replacements if nothing else fixes the problem.
Air filter looks consistant with about 5000 miles motoring - certainly not bad enough to stop the car from firing up, but the car won't start with it removed either - so no joy there !
Fuel filter, I don't know - I've yet t find it
plugs stink of petrol after trying to start the car so I guess fuel is getting through ?
Fuel pump runs for a couple of seconds when ignition is first switched on and, if the engine has actually run (a rare event) then removing the fuel filler cap gives a noticeable 'hiss'.
I also checked that the throttle switch was operating okay as I guess this somehow affects the idle circuit. Closed with throttle and open-circuit when throttle depressed. Voltage at the plug that goes on the switch, with ignition on, was 4.65 volts.
Acting on a rumour that the four-terminal coolant sensor might be faulty I checked the rsistance across diagonal terminals. 4620 ohms one way and 4590 ohms the other, at today's ambient temperature -I'd guess about 10 c.
So I'm still looking for the fault and trying to avoid throwing money at it without knowing what is actually wrong.