I've tried replacing the cadence sensor batteries and re-pairing with the garmin and it still does not work.
Research suggests that I probably need to reset the garmin to get the cadence sensor working again (which will loose all my settings and data).
If the malfunction is due to a corrupted file then perhaps I can get it working by copying the relevant system files back to the device (from a previous backup copy) without doing a reset (e.g. overwriting them with an older version)? The problem is I don't know which files are relevant to this problem.
Any suggestions?