Thanks for the pointers on this. I spent ages reading around and decided to try the crank sensor first. The new one has been on for 2 weeks now and car is starting fine.
I did have probs fitting the new one though. Despite the same part number stamped on the sensor it was subtly different in so far as the cable exited the sensor mid way down it rather than at the end. This caused it to foul the the block. The sensor could be pushed fully in, but only when it was turned slightly causing the securing hole to not align with the screw hole in the block. I had fiddled around for half an hour before I realised this and the new part was out of the bag and grimy so no chance of return. My fix was to slot the hole so it would line up and be fully into the block. Not too happy about having to do this, but retained some of the metal strengthening sleeve which clamped ok under the washer. As there is no force on the sensor I am happy to run long term with this although it is not ideal - worth taking the old sensor with you and carefully comparing. I could post a pic if anyone wants to see one.