Hi Guys
Thanks for your responses. Yes the TCU (J217) is externally attached to the top/rear of the gearbox and looks like the left hand photo at
http://www.ecutesting.com/vw_and_aud...arbox_ecu.html
This module includes, among other components, the multifunction switch - F125, and yes you've guessed it you cannot buy this separately.
These are the guys that quoted me the c£250.00, and with 2 years warranty is probably a better deal than a new one from Audi.
It could be co-incidental that this all kicked off at the same time as the water ingress issue, but take I take your advice about inspecting the wiring harness, before spending big bucks.
It is clear that the start authorisation module is getting unclear signals and is (correctly) blocking the engine start.
Later today I will get the diagnostic report and recommendation from Audi, then I will decide what course of action to take. I have located an independent Audi Specialist about 5 miles from me. It seems likely that I will be giving them a visit tomorrow.
On top of all this I have just remembered that the MOT is due on June 2nd.
I will keep you updated as things progress