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Just to update as the final solution may be of interest to anybody with this problem and may save them a small fortune. The auto-electrician diagnosed that the pump was taking around 80amps hence why the fuse was blowing. The prospect of removing the pump was quite scary but in reality it wasn't too difficult. Once the pump was removed I took the back plate off the pump motor and I could immediately see the problem, around the electrical brushes and commutator there was a horrible black paste, it looked like a mixture of grease (from the bearing?) and carbon dust produced from the electrical brushes as they wear. I cleaned this up and ensured the commutator and brushes were connecting correctly (the brushes still were more than 50% of their original length at 100k miles) put a small dab of copper grease on the bearing. Reinstalled the pump and it is now working as it should be. The whole job took 3 hours and cost £32 in fluid from ebay (then add £60 to that for the auto-electrician). I would guess that many pump failures will be caused by the same thing, it is well worth taking the pump out and checking insider before going for an expensive new pump replacement.