I think I have finally tracked down the problem. The terminals on the wiper motor were loose. I am assuming they kept touching causing a short and blowing the fuse.
I have replaced all the terminal connections on the wiper motor and (touch wood) , it hasn't blown since.
The moral of this story, don't assume it's something you have done and carry a bag of 35 amp fuses. Thanks for all your input, I've learnt loads going through the different theories. Steve