Just responding to this in case it comes up in someone's search later looking for a potential solution to a similar problem.
Firstly thanks to those that replied, I had gone out and tried a few other things to diagnose the problem and figured it must be related to the steering wheel switches as everything else related to the transmission was working fine. Then an unexpected and helpful eureka moment, the horn didn't work. So thought bingo, must be electrical.
Followed the steps online and removed the airbag to get in to the wiring underneath to see if there was something obvious that could be the issue, and luckily found that one of the wires for the horn was worn right through and hanging on by a little bit of the plastic cover.
I figured as it wasn't fully disconnected there must have been some times when the circuit was completed hence things working previously but intermittently. Anyway the wire quickly broke away completely so I pulled the spade connector out, stripped a bit of the wire back and fixed to to the connector. Put everything back in place and now the horn and shifter buttons work as they should.
Anyway will see how things fare with the fix for now, but massively appreciate the forum and this community for the resources/advice so I could (fingers crossed) sort this out.
A couple of quick things for people doing similar tasks (that I got from other posts but helpful to have in the same place as the above):
- the disconnected red/yellow wire in the last photo is supposed to be disconnected, so don't worry that it isn't where it should be (not sure how model specific this is, but mine is a 2003 S)
- don't do what I did, which is diconnect the battery and then close the bonnet!
That was a split second moment of stupidity, but at least taught me how to release the bonnet with jumper cables and the connector in the fuse box