hi, if its not fuse holder or wiring could be...
doubt its the battery if its operating on/off, if it was the battery it would just operate slow.
its more likely the pump itself or the high pressure switch. is it wired in?? should be two wires on top of pump
dont be pressured in buying a new pump, they are very strong, instead buy a new head and pressure switch roughly 25 quid, very easy to fit just 3 screws.
williamspump 0845 226 3638