Success!
This update is mostly for PlasticMac's benefit but others may be interested.
Received a used replacement siren, 8L0 951 605A, same as the one fitted it to my Mk1 TT. My TT is 2002 vintage so the CANBUS protocol is the same as my A2. Checked it actually worked by briefly connecting to the TT. This can also be done by carrying out an Output Test on your A2 if you have a competent scan tool. You may also have to clear a confort module fault code from the original siren failure.
The afore mentioned part number is not my TT's original incidentally, but an updated one that was fitted to later Mk1s. I fitted it years ago after the original gave up. Do not try later part numbers as they won't work, even though they will look the same. The protocol is different on later sirens.
Removal of the old alarm siren, without pulling trim off, is not possible. There's literally a 1mm clearance deficit, so I have left it in place and mounted the replacement alarm on the soundproofing foam with double backed adhesive pads after making a hole in the foam to locate the siren's mounting stud. In the long term I will mount it in place of the original, and fit a new battery while I'm at it.
If someone following this procedure knows their A2 used to give the audible single beep originally, then there is nothing more to do. Their A2 should now beep as previously, i.e. before their siren packed up. Equally, the siren should now activate when the alarm is triggered.
In my case, I now locked the car to listen for a beep when the indicators flashed. No beep meant my A2's Comfort Control Module had never been coded for the siren beep in the first place. I have an ODBeleven Pro already (15 years of Mk1 TT ownership will teach you many things!) so I was able to code the function into the CCM, by adding 01024 to the existing value and carrying out an Output Test.
So, for the price of a used part (£25) I now have a functioning alarm siren for deterent purposes AND the audible acknowledgement, on remote locking, that the alarm is armed. This may all sound like a bit of a faff but is actually quite straight forward and piece of mind is a wonderful thing for £25.
The foot note here is this. If you have a functioning siren still, it WILL fail soon. The back up battery will fail and leak, causing irrepairable damage to the PCB. If you do not have a functioning siren it is because this has already happened. New sirens (8L0 951 605A) are available on ebay for around £140 plus customs duty (somone in Latvia has bought the stock). If you have basic practical competence, my advice is to buy a used one that is still working and, at the earliest opportunity, open it up and fit new batteries.