Thanks for the replies, I had heard about the suction valves, was not aware they could be changed in situ, nor of a supplier of them. This vehicle already has clear fuel hoses from the filter to the pump, and also leading into the filter, before and after the hand primer. The filter and housing were changed because there were a few air bubbles in the pipe from the filter when the engine was revved, but not before the filter. I have ordered some clear fuel pipe for monday, I don't have a long enough length to be able to check whilst driving. Will also tee-in a vac gauge to check for a restriction in the fuel feed pipe or tank pickup, will also be able to see from this gauge if the suction valves are able to suck!

I still have a small doubt, due to having no fault codes, as if the engine is not requiring high fuel pressure e.g. intervention of traction control system, but can see no evidence of this. Should have a better look Monday.