Martin, I did have an aftermarket valve, same as you, made it much worse too so old one back on and cleaned. Didn't have much luck sorting it out myself so it's going away to be looked at. Fuel pump is on the list. I'm thinking new pump filter and wiring could sort it. Luckily all the rad and fans etc has been renewed fairly recently, even the metal pipe under the rad is new and shiney.
Need an ABs sensor too but can't find one for love nor money ( I've offered both too) What does everyone else do? Understanding Mot tester and live with the light on?