Great, thank for the advice!
I'm now heading for a few days holiday but after that I will try to play around with the switches.
I'd suggest contact cleaner too. With the ignition off and after opening a door so it won't operate, squirt it all around the switch so it runs inside then give it lots of switching, to hopefully rub/wash away the muck. After leaving it to dry for a few minutes, switch on the ignition and give it a try.
I don't know what the electrical properties of WD40 are - if it insulates then it might stop it working, but if it conducts then that may be worse. The point of contact cleaner is that it evaporates completely really quickly, leaving nothing behind. WD40 leaves oil behind after the solvent has gone.
Contact cleaner is normally just a liquid form of a gas - butane, propane or a mix. You can also use lighter refill gas. Obviously no smoking either way!