Currently, on the phone to Apple Support - we've gone around the houses verifying everything and ensuring that I qualify. On hold, while the operator consults with higher-up types..
Well, that went well - not terribly productive at all. As my Watch was bought through my carrier on contract, I have provide proof to Apple that I purchased it. Alas EE in the UK have never been great about documentation and even their own bloody web site gets the bloody device wrong and still lists it as the series 4. This, despite having purchased bloody AppleCare+ for the sodding Watch Series 6 device, Apple cannot issue any kind of coupon for the trial.
So, I have to go through to EE and get them to provide some evidence that I have bought the device through them via contract so I can prove to Apple that it's my device. Technically it's not my device, of course, but I'm sure you get the idea.
And I've been on the phone with Apple Support for over an hour.
All this for a bloody three-month trial.
Don't think I'll bother now.