Took it to bits tonight, the centre tube had a small split in it which was enough to let water into the mechanism and seize the rotating centre section. Motor is working fine so the repair kit should work just fine.
Interestingly enough the water jet onto the rear window was still working, so the split wasn't enough to actually stop the operation, but water and rotating metal parts dont mix to well!
Thanks for the good advice, the real benefit of forums such as this!
regards