405 Mi16 slow to start....... fuel pump maybe??

I guess you've probably got an electric rather than a mechanical pump?

If so, you can sometimes hear these kicking-in.

leave the car overnight, turn on the ignition (radio off!) and listen carefully - you should hear the pump start up then maybe stop after few seconds as the pressure builds-up.

Other 405 owners could put this to the test and let you know if this trick works.

Also, I would expect that if the pump was faulty you'd get fuel starvation at high speeds if fuel injected.

next - does the car have a carb instead of injectors? - is it leaking/float chamber draining etc? Does it have an auto choke which is playing-up?

Might be worth looking at some of this stuff .... PS I'm assuming you've done the usual plugs etc. checks

Yep I've tried to listen out for that sound just before the starter motor starts but I can't hear anything. Anyway I thought that priming wasn't necessary on this car because the non-return valve in the fuel pump stops fuel draining out of the system??? The engine is totally standard but had a full rebuild 10k miles ago purely because the bottom end was getting rather rattley. See, if i crank her for a few seconds and then stop before she starts and then do that again, and then try to start her up fully she jumps into life, which indicates to me that there's no fuel left in the lines.

It is possible that your non-return valve is faulty, but this wouldn't take

I'd check the injectors and plugs.