So are we to rely on Mrs T to decide that my driving has become a bit 'erratic' or what?
Yes.
My step dad has dementia, but hasn't been banned from driving. I went out with him to evaluate his driving a few times, and it was decent. Having dementia he's forgotten a lot of things, but driving hadn't been one of them. He could still remember the way from his place in Mid-Sussex down to ours in Hove. It's just that he'd do it at 4 in the morning because he was confused.
We informed the GP and the DVLA a couple of years ago, and he's in the system, but it never got to the point where they said he had to stop. We have now stopped him from driving as he's forgetting too many things.
You can get tests from specialists, but you either have to pay privately or wait for ages for the GP/DVLA to sort it out.