I have now managed to replace the plunger with a pattern part (from Sparex.com). I did have to grind the end down a bit as it did not fit very well into the fork on the linkage so that the clevis pin would only go through the assembly when the plunger was at about 30 degree angle.
That is now sorted and the top half is back together. However, I think there is a problem with the control valve as I cannot seem to push it back into the pump and it doesn"t seem to be in the right position to be re-united with the control linkage. Has anyone had a problem with this before? They say a picture paints a thousand words so rather that me going on and trying to explain I have posted up some images in the hope that this may demonstrate the problem and someone may know the solution!
Thanks in advance