UPDATE::
So this is a bit of an update to my speedo problem. On the way home tonight it started to come back to life. I had checked the cable between the sensor and the point at which it disappears up past the front cowling checking for tight spots, damage etc. Sure there is a tight bracket on the fork (near-side) that clamps the sensor cable as it snakes up into the cowling, but that has always been like this. Not sure if these were factory installed but there are cable ties helping to keep the cable in place as it makes its way up from the fork, over the brake pipe 'cluster' and off under the cowl.
Anyway, it didn't start working in a smooth "bang! there it is working again" sort of way, instead it worked then stopped, waited a while then bounced around for a bit and then started working again. Then it would stop dead and do nothing for a while before starting the whole thing all over again. By the time I neared home (30-40 minute trip) it seemed to be working as expected. Will need to see if I can take her out again tomorrow for a test to see if the speedo is still working.
My understanding of how these work is that they are electrical sensors and there are no moving parts (in the sensor at least). So I am not sure if there is anything that can seize and then loosen off later.
As before, any advice would be greatly appreciated.
Des