Just checked it again and the magnet was ok. I rechecked the settings and the wheel size seems to have switched to auto. I've changed it to manual and put the recommended wheel size in and it looks like it's sorted. I'll go for a ride or get on the turbo tomorrow to check.
They do fail after time, as do the H.R.M. I seem to get two years for a Sp/Cad sensor and about 18 months for the HRM, my current Sp/Cad, I can pick up the cad on my 705 but not on my Rider 20, and no speed on either.