Yep the diaphragm goes in the FPR but isn't that common which is why it wasn't found quicker on my car - but then you have to remember they are mostly 26+ years old now

The problem is fuel going down the vacuum line back to the inlet

The process to rectify it is to change the FRP to start with then remove the inlet and clean out - new inlet gasket etc i did just for matter of course then flush the engine through with at least 8litres or 2 x complete oil changes with a couple of cheap filters to remove the oil/fuel from the engine - then fill up with good recommended oil and decent filter and your away again with an internally clean engine

Sounds like a big job but it's an afternoons work really