I've subsequently found an A8 Parts Forum thread from early 2020 when someone took a slightly different approach to this problem, though they did use gasket sealant as part of the solution.
See: https://forum.a8parts.co.uk/showthread.php?t=15457
Their instructions overlook the two screws in the boot ...

I wonder if a few dabs of suitable adhesive would be enough to seal that? :roll:

Well I tried my "go to" product and, for now at least, it seems to have worked after leaving to cure for about an hour.

For reference: KISS Powerflex Brush-on Nail Glue. :mrgreen: As used by SWMBO.
It's effectively ...

The culprit! BootLock.jpeg
When unlocking (which I can confirm is SUCK) this closes up somewhat so that generally works. The input pipe is fine.
When unlocking it just blows air out until the pump times out.
Incidentally using the valet lock just physically stops the rod moving which due to ...

Still not looked at it, but I heard it running on for quite a while this evening. When the pump stopped the boot was not locked, pressing the button opened it (and triggered the alarm of course).
Interestingly putting the key in the boot lock and turning it clockwise (to valet mode) then back again ...

Stevenage isn't too far for me, what's the condition/type/quantity of the FE wheels, e.g. still diamond cut or repainted, genuine 8.5" or later (D3) 9", welds/repairs, tyres etc. and what do you want to swap them for?
Your original ad mentions PS4S tyres, what's the date code and wear level ...

Stevenage isn't too far for me, what's the condition/type/quantity of the FE wheels, e.g. still diamond cut or repainted, genuine 8.5" or later (D3) 9", welds/repairs, tyres etc. and what do you want to swap them for?
Your original ad mentions PS4S tyres, what's the date code and wear level?

Last Friday he was here. Strut Eater 5000...

Would the coding of the RNS also need changing - depending on the amplifier used?

I won't be the first to ask; what a VCDS scan shows? Without that no-one has a clue how bad the gearbox problem is.
Also, will it actually drive in limp mode (PRNDS lit) as that usually means its locked in 3rd (4th?) gear but still drives.

must be something thermal surely, but what? :twisted: The control unit is in direct contact with the underside of the sunroof so gets very hot very quickly. I've inserted a couple of felt pads to give a few mm of spacing.
I also noticed the system still works if the sunroof is left open tilted ...