Well that didn’t goes as expected.
I took the top off the fuel pump, only dropping one allen key into the depths of the engine bay, marked the diaphragm against the pump body with a marker pen and removed the diaphragm and attached boost pin to reveal this.
I removed the spring to clearly see the whole the fuel was leaking from and it is the black plastic hole on the other side of the pump.
For fuel to be dripping out of there it must be getting into this top part of the pump where it doesn’t belong as WelshFarmer said. The boost pin looks clean and undamaged which means it must be the seals around it that are letting the fuel through.
With a sad heart and £180+vat bill looming I put it back together carefully aligning the pin and diaphragm with the markings I made. I started it up to make sure it still ran and looked at the drip – IT HAD GONE. I revved the engine and nothing, not even the smell of diesel.
I am guessing some crap from dirty fuel had got sucked in and was stopping the pin seal properly or something. Whatever, I don’t care. I fixed it. I am claiming the win!