Right, I've got some life out of the minus key. Spamming it furiously gets a result 1 in 100. So, I'm going to strip the switches out and pull them apart. At worst, I'll have to buy two new switches and fit those instead.
Update: Fixed!
Was a combination of several things; lifted/broken tracers and the contacts inside one of the switches not making contact.
The two blades inside the housing were not making contact so I bent the contact out 90degrees and then bent it back. It presses against the plate fine now, just feels a little soft when used and doesn't make the 'click' sound any more. I'll replace the switch at some point but I'm not hugely fussed.

On top of that, both the '*' and '-' switches needed rewiring.
