Did a bit of investigation work last night with the help of a friend.
Firstly, there is no bleed nipple on the filter or housing, the top nut holds the filter and housing in place
. The bleed nipple on the pump (red arrow) when loosened slightly jetted diesel as if under pressure.
Stop solenoid (yellow arrow) is what I replaced.
Blue arrow is, I think, the fuel return pipe to the tank.
It seems that there is air getting into the system as the plunger on the fuel filter goes soft after a run as it needs 1 or 2 pumps to become stiff again. My friend thinks that the air is causing there to be extra fuel to be pushed through into the cylinders. However I think it may be air rather than fuel at the injectors.
More investigation to go on over the weekend.