replace the pump piston and seal kit 150.00.396c it will be letting by. if your fluid in the pot is dirty this is a good indication as well that its knackered. done mine today with the same symptom as yours.
Just found this on amazon its richard hammonds new dvd and features alexz wigg riding in it.
Theres a small clip showing some of the dvd.
Looks good might ask for it for xmas
EDIT: Link sorted, Forum Mod