Thank you for all the replies. I have had a little time over the weekend to investigate further, I removed the air-filter, back plate and throttle bodies so that I could better see the rear of the water pump / thermostat. A fellow member who built a similar powered car a number of years ago suggested I removed and inspected the thermostat but even with the above removed am struggling to find it. The bottom (large) hose from the radiator goes directly into the back of the water pump (with a tee piece off into the expansion bottle), the smaller hose from the water pump goes directly to the heater control valve. On the return side, the output from the heater goes via the diverter valve down into the solid water rail with a sender at the rear of the engine (brown multi plug) then another sender (single wire – gauge sender?) then into a large hose into the top of the radiator. There is another small hose from the expansion bottle back into the inlet manifold which I have removed so as to clean the non-return ball valves (thank you Silver Machine) – it did seem to be gunged up. I’m now intending to pressure test the system again now that it is drained from coolant (to eliminate a false reading caused by an airlock) but would like to find the thermostat location is this contained within back of the water pump? One final thing – I assume unrelated, by the inlet manifold between cylinders 2 and 3 there was a foam air filter pointing down, this had disintegrated so will replace but was wondering what the correct term is for it (picture below). Once again, thanks for all the help. Chris