I had this problem as well. Turned out to be a dodgy connector for the fuel pump that sits behind the number plate surround, same one that does the rear numberplate lights iirc. Took aaaaaaggggges to find lol.

I also removed the bulkhead loom plugs - If you are doing it, join on wire at a time - Don't be a hero and chop the entire plug off or you'll be there for ages trying to figure things out

In the end I replaced both the engine and fuel pump looms for peace of mind!