Latest - I've fitted the aux cooldown fan with a capillary thermostat strapped to the fuel rail.

If I set it to 55C it stays off whilst driving the car .... and kicks in when the car has been shut down after about 3 minutes.

Slight problem, It stays on for about 40 mins whilst the engine cools and I think its drawing about 15 amps - so possible flat battery issues on really hot days.

I've adjusted to 70C and it still kicks in on shutdown .... waiting to see how long it stays in for at this setting.

Next up, Ive ordered a fuel pressure measurement kit (surprisingly cheap @ £25). There's a schrader valve test point on the fuel rail so I'll be looking for about 70psi engine running and paying particular attention to pressure decay on shutdown.

What I can't explain is that it starts on the nail when cold but cranks for ages when hot.

If it were a faulty check valve (apparently there's one as part of the in-tank pump unit) then why only a problem when hot ?.

Hols now for a week - I'll be back on the case on my return.