Engine: 70 R: Non starting following shut off

[S80Dave]

Didn't know you could get a cheap smoke tester.
I have checked the hoses for leaks but with a home made contraption. I used an old hand pumped garden sprayer. Put a Plumbers smoke pellet in and pumped some smoke into the induction hoses. You only get about a minute before it burns out and starts burning the sprayer! Possibly not a recommended method but it showed a slight leak now tightened.
Alas not the problem, nor was it the crank position sensor which I replaced yesterday.

Also I have checked the throttle position sensor outputs in case it was showing slightly open when closed and asking for fuel when restarting. About 0.61V when throttle closed (or idling) which I believe is about right. Rises up to 4 or 5V with throttle and settles back quickly to 0.6V when throttle off.
Fuel injector connections all look sound, and I have fuel pressure and fuel pump running when attempting restart.
I have yet to check for spark when attempting restart but will do. Still puzzled as to why this condition is only present during the first few minutes of shut off. Engine is still surely fully hot at 6 to 8 minutes after shut off and will start normally, so not really a 'hot start' problem.
Will update if there is any progress.