What you are suggesting is exactly right, I have already removed and cleaned the fuse box and have just come back in from refitting it back to the car.
There is a black box piggy backing the fuse box, which contains a PCB, I completely stripped both the fuse box and the PCB of all relay and fuses and there was evidence of water ingress on the PCB. I used some Vaseline and cotton buds to clean up the components and put it all back together. Fitted it back to the car and the wipers are now working again.
Many thanks for taking the time to offer advise.