I have a strange problem, reversing lights and sensors not working. Checked gearbox switch, earths, bulbs and fuse, all OK. Remove the reversing light bulbs from their holders and the sensors work. Replace one bulb with a 4 amp bulb and it lights. Put back 2 perfectly good 21w bulbs and nothing works. Any ideas? 

The wiring to the parking sensors and the reversing lights are connected in the boot behind the cover on the passenger side.

Unfortunately this is also the routing for the rear washer pipe and the coupler in the pipe can be forced apart by freezing if insuficient strength washer fluid is used in cold weather. The result is water getting into the connections causing poor contact due to corrosion.

Take a look and see if there is water in this place. If so dry and clean the appropriate wiring and then carefully check the connections, including any earth points, and of course reconnect the washer pipe if leaking.

As above. Clean up your earth points with a bit of wire wool/ sand paper, so nice and clean and shiny. Cover in Vaseline or WD40 to keep moisture away. It sounds like a 'floating Earth' problem.