All done and working perfectly!

Not quite as easy as I thought it was going to be, with two unexpected issues.... firstly there is a heat / rain shield to remove before you can access the multi-plug (or at least there was on my car):

And once I'd unplugged the multiplug I couldn't release it from the car at first. (It's clipped in and simply prises off). I couldn't get it to slide back onto the clip when I went to refit it, so secured it with a cable-tie.
This is the inside of my switch:

There was some water which dripped out when I separated the halves - hopefully this was the culprit. I cleaned it all with maplins switch cleaner which didn't reveal any obvious damage. Then lubricated with silicon grease. For some reason I expected this to be the same white grease that was originally inside the switch; instead this was a very thin, clean liquid. Is this ok?
Any once cleaned up it all looked like this.

I ran a bead of vaseline in the edge where the rubber o ring seals the two halves in the hope that this will ensure it all stays dry inside for the future. So far all is working perfectly again..
Thanks as usual for the advice