OK, have to confess my embarrasment since, as so helpfully pointed out above) the fault may have been there all along, but without having paperclipped it before I started we will never know.
Note to self...either do it properly in a logical sequence (like in those procedure documents I spend large parts of my day writing) or give it to someone else to do.
Worry not though, the crank sensor is not hard to change 
It may be a relatively simple job to change, but do not be surprised if you end up having to simply cut as much as you can reach off each end of the cable from the one you're taking out. The way that they were fitted to the engine before it was dropped into the car is a bit a fault in my opinion and in most cases there is not a cat in hell's chance you're going to pull the old one through and out if it is the original one as was fitted at manufacture.
Leaving the residual (trapped) cable in there is of no consequence and the guide on here has useful advice on much better routing. Make sure you keep the new cable clear of the exhaust which is a common cause of recurring failure.