Replaced battery in my VW Golf GTI, since then.

You got caught. Volkswagen is one of the top three manufacturers in the world for designing in tricks to cost you money after the sale. Since the late '90s you must never disconnect the battery or let it run dead. If you want to replace the battery on your own, you must use some type of "memory saver" device to maintain the many computer memory circuits. On most of their cars the engine will not start, and on those that do, most of the newer ones won't come out of park and engine speed won't increase above idle when you press the accelerator pedal. They have to be dragged skidding onto a flatbed truck. Since you didn't indicate yours won't start, take it to the dealer to have any locked-up computers reprogrammed.