[jimh]

Update - No better. I still got gearbox errors when cold / damp - only at startup.

Today I replaced the switch with a cheap one I got from Ebay. New, but probably Chinese copy, shipped from USA all in for £35. Ebay item 282315422285

The cable is slightly shorter than OEM, but it just reaches if the connector is not clipped into the bracket. It's not going anywhere though - it's too tight.

Things I found made it easier:-
Loosen bottom 2 bolts at top of exhaust downpipe. Remove 4 bolts downpipe-cat.
Remove headlight level sensor.
I never replaced the top bolt on the CV joint heat sheild, but the remaining 2 bolts are pretty huge for the application.
Remove 2 bolts gearbox mount - subframe.
Remove 1 bolt gearbox mount - gearbox. Only the one nearest the switch.
Loosen the other 2 bolts gearbox mount - gearbox as I found these awkward to get started. Wedge screwdriver between gearbox mount and gearbox right where the removed bolt went to hold the mount away from switch.
Jack up gearbox to replace gearbox mount bolt.

I don't think the style of switch I had was an improvement over the one most others seem to have. Although mine looked more robust, I think the "sweeper" style switch is self cleaning when the one I had was not. Although the sweeper will gradually wear out, the other style will get tarnished contacts which are not cleaned in operation.
We'll see how long that Chinese switch lasts...