Good evening. Belatedly, an update to the thread.
Tom -
@2work - very generously invited me over to Coalville, where he offered to swap both my hazard relay and my alarm siren over with known working parts, and I drove over on Saturday morning. Thanks to his expertise, my hazard relay has now been replaced, but after further discussion with Tom, the alarm siren has been left in situ. I made this (possibly mistaken) decision because the existing siren clearly still works, because it never shows up as a fault on VCDS unless my battery has run flat, and to a lesser extent because I sensed that other participants in this discussion were moving to the same view.
Tom ran his own VCDS checks on my car, both with the engine off and with it running, and nothing relevant showed up at all.
We spent the rest of the morning doing a few lesser jobs on my car, and then I had a look round Tom's impressive fleet, before heading back to Norwich.
So a
massive thank you to Tom for everything, his time, his company, his hospitality, his knowledge and his practical skills.
Because the battery in my A2 has been flattening at irregular intervals, it will be quite some time before I know whether or not we have solved the issue. One other possibility is parasitic drain on the battery which is not that easy to test for when the car is parked on a public road. However, when I have checked my battery with my multimeter, it always shows as fully charged.
Thanks once again to Tom, and to everyone else who responded, both here and in private messages. All your advice was helpful and appreciated.