Pete
Had this problem recently on both the 400psi and the same week on the 135psi machines.
puchased a pump head repair kit from ashbys for the 135psi relativley easy to fit, just watch for the springs popping out when you take the head apart(about 15 quid) and for the 400psi purchased a new pump head as the water was passing, not as easy to change due to lack of room (about 80 quid i think) i also changed the seal on the pressure regulator on the 400psi as it was dripping slightly.
Hope this is of some help.
Regards,
Gary