Update on investigation so far.

I have checked all the pin outs and the groud one is going directly to the fuel pump feed.

I have set it all up correctly now and wired it back to how it should be and the noise has stopped BUT ... It is clear this was done because of an issue as it now primes fine and stops but does not turn back on when the engine is cranking so won't start.

Fortunately I have wired it in such a way I can quickly override this to how it was bypassed originally so I can still use it, the investigation continues.

I'm going to try replacing some of the components, starting with the fuel overrun valve on the airbox, reason for that is it was unplugged when I got the car. I will work my round the components if it is not that.

I'll post an update when I have one but if anyone knows what this is before, feel free to let me know.