That is a frustrating pickle of a problem for sure. Sorry to hear that. The idea of spending that kind of money to replace the light with basically no guarantees is definitely not very palatable. It is a shame, because it is a beautiful car.

It's really puzzling though that nobody who's gone through it managed to find out from JLR what's leading to the fault. I mean, it's a headlight so nothing really advanced. Is it a driver board issue, is it a grounding issue? The Mk2 TT had a similar issue with the rear lamp assembly due to the original ground not being man enough leading to a slow burning of the connector. The indication that you had this issue was that the brightness would become dimmed. Delay the repair and you're looking at a new assembly AND wiring which could be a pain to replace. The online community then came up with instructions on how to resolve the GND issue and even an aftermarket DIY kit for £16 put together by another TT enthusiast.