Water Water everywhere!

[A8 Adie]

Hello fellow members

I’m a newbie to this forum though I have been following some threads for a couple of months now.
I have a 2004 3.0 V6 A8L multitronic with only 44.5k miles on it – no sun roof.
I read gninnam’s recent thread ‘Testing the water’ with much empathy and I must share my sorry story with you as concisely as possible – which is still ongoing.
My plenum drain holes were clogged up such that the drivers and nsr foot wells were very wet.
The electronic handbrake fault light lit up as did the PRNDS lights. Car would not start. Called out a local auto electrical garage who has a VAG-com they reset all the fault registers and the car fired up – Hooray!
Not knowing where the drain holes are (then) I took the car to local Audi dealer and instructed them to clear them for me. They charged me a very reasonable £65.
Three weeks later I soak tested the front of the car before planning to replace the now dry carpets. This I did with a hosepipe directed at mains pressure only at the top of the windscreen and sure enough the water was draining under the car at the correct places. QED the plenum drains were clear. I then took the car for a brisk 2/3-mile drive with the AC fan on full. On parking up no water had been drawn through the heating vents at floor level…Nearly there.
2 hours later on returning to the car the original fault was on (handbrake and PRNDS lights; car would not start). Called out the same garage and the fault reported was ‘park/neutral switch intermittent short to ground’ and a fault code 17xxxx something reported on the F125 multi-function switch. After clearing the faults the car fired up.
Thinking I had been a little heavy with the water I did nothing else. The car has been OK for 3-4 weeks even though we have had one or two heavy downpours. On Tuesday 20th the same fault happened even though the previous 7 days had been gloriously fine, dry and hot.
The local Audi dealer is quoting me £989.50 incl vat for the TCU (J217) which together with 4/5 hours labour, replacement oil seals, gasket and fixing bolts – maybe replace the ATF whilst we are at it -is going to cost me somewhere in the region of £1600-1800! Is there a better way? Has anyone experience of ECU repair services? I’ve found a site quoting c£250 on a 5-day turn around. What about a second-user unit from A8 Parts? Or following gninnam’s inclinations do I get rid of the car?