Problem is now solved but I'll stick some info here which helped me, in the hope it can help someone else in the future.
Had a check of the ECU / plenum area - as I hoped (and expected) there was no sign of water ingress or flooding.
So , following some extensive late night reading, I came to the conclusion that it was a K-bus fault somewhere as the symptoms were all related to items on the K-bus.
I found
this thread very helpful - specifically the last post from MGMarc which details a simple diagnosis test for each of the K-bus lines. My plan was to locate the connector in question (which was tucked quite high up behind the RH side of the lower dashboard - up and behind the light switch module), remove each wire in turn to see if the communications were restored, and then refer to the wiring diagrams on RAVE to determine which component was throwing the circuit out.
Due to the location of the connector on my wiring harness it was quite difficult to access without being a contortionist, so party due to laziness I thought I'd start with some of the connectors on the other ends of the K-bus lines. I haven't got a FBH (which would have been my first port of call), so my first stop was the CD changer in the boot.
I pulled off the little hatch in the boot and sure enough there was a pool of water with an unused connector sat in it, one wire of which was white/red/yellow (K-bus)
A quick dry off of the connector, and a disconnection of the CD changer entirely, and hey presto - all systems returned to normal.
My next stop would have been the seat connectors which are prone to corrosion and getting wet if the carpet under the seats are wet.
Next task is to figure out where all the water is getting in from!!