C6 estate auto box woes

So my old 200k+ miles '05 reg 3ltr diesel estate lumbers on but has now thrown what looks like a gearbox problem and now won't select gears. The centre display went bold and diagnostics indicated fault numbers P0729, P0735 and P0783. Initial research suggests either the ECU or the transmission itself - obviously I'm hoping it's the ECU but I can't find conclusive info as to where it is? Some suggest it's under the carpet on the passenger side but is that l/h or r/h drive passenger? I'm going to try to have a look this weekend (if weather allows). I did have a problem a few years ago with the heater matrix leaking coolant into the cab so I'm wondering if the ECU is under the floor whether that might have had some adverse effects, but why has it waited all this time to show itself? Oh, one more clue, my diagnostic box could not communicate with the EMU and that's new so would gearbox ECU failure break the comms chain and cause that too?
Btw P0729 is "Gear 6 Incorrect ratio", P0735 is "Gear 5 Incorrect Ratio" and P0783 is "3-4 Shift".

It is in the sump of the box itself. ZF 6HP-19

Hmm, that does't sound like it makes things easier ..... Interesting write-up about that gearbox here: ZF 6HP transmission - Wikipedia maybe I'm lucky it's lasted 200k! So is there any hope or does this mean the old girl is heading for the breakers? I spoke to a local Audi chap who said he'd be disenclined to change the transmission fluid in such an old box, it nmight cause more problems than it solved, but that was of course before this catastrophe!

Personally I would say it is terminal as you were selling it anyway, you would be looking at £3000 ish for a rebuild at a typical specialist or if you had really lost touch with reality, £6945.86 plus installation for a genuine Audi exchange unit or you could really lose the plot and buy a new one outright for £8993.18... but those prices do include VAT!

Ok so assuming I don't have that money and haven't got a winning lottery ticket scrunched up in a pocket somewhere, would it be worth trying a fluid change? I can't see that there would be much to lose at this point.

I think that would be a few more hundred pounds into the drain unit, at least it would make the job of stripping it down cleaner. This is going to be a problem that kills good useable cars in the future even more than now, £20k for a battery at 10 years old may concentrate the mind but few are listening and the consumers of the future are going to pay the price… oldies will blame Boris, the younger ones will say “who?”…

Oh don't get me started on the EV subject - even ignoring the whole battery issue (which is controversial enough by itself), what happens when everyone gets home at around 6pm, then everyone plugs their cars in to fast recharge (so they stand some chance of going out again that evening) and then many try putting the ovens on too to cook dinner? I see pylons melting if the grid hasn't already tripped out!

Anyway, back to the subject. It's strange that this fault has suddenly appeared, the vehicle has only just had a successful MOT after having some work on the coolant/heating system (split pipes) which wasn't cheap and was running fine until literally 2-3 days ago, so I'm loath to give up on it just like that. I'm aware that throwing money at it would solve the problem but I don't have a lot so I'm going to make sure every avenue available to me is tried before calling ASM etc.

I've just done some more investigations and here's what I've found:
1. The vehicle still won't select a gear, but since I cleared the fault codes a couple of days ago, they have NOT reoccurred.
2. I disconnected the battery, left it so for a while and then tried again. Still no gears but still no fault codes either?
3. The gear select lights next to the gearstick, and the dash display, all appear to be doing what they should, in both auto and semi-auto modes.
4. I can read the transmission status fine but cannot communicate at all with the engine ECU. This is definitely also a new problem.
I can't help but feel the gear selection fault and the engine ECU problem are related somehow? Could failure of one or other, or failure of wiring/connectors between them, have any bearing on the situation? Is it really true that the gearbox ECU is actually inside the sump of the gearbox?

For what its worth, my Touran TCU ( its a TCU for a gearbox, not an ECU ) is behind the near side headlight, under the wheel arch liner.