Update: New injector harness fitted. No dice. Master tech took the car home for overnight road test last week, but the fault maintained. Radio silence until today, when I phoned them for an update. Told that a new ECU is now required, as it's "blown" - as they do

ECU is on backorder, so won't know until Friday when this will be available. Been without the car for 5 weeks now. Fault reported coming close to 5 months ago. I guess I'm just super lucky

What is a little worrying, is that VW UK are supposedly remotely diagnosing the car at this point, and they clearly don't know what's wrong with it. I suppose it's possible that a component failure in the ECU could be reporting false readings elsewhere in the car, which is why this feels a bit like a trial and error diagnosis. If a new ECU doesn't fix it, then I think I'm just going to ask for a replacement car.
Cannot recommend this experience.