Thanks. As it happens, I have some time booked with a different instructor next week, so I'll see what happens.
But really the problem is that no instructor has even remotely hinted that I won't succeed. I'm just comparing my (unremarkable) progress with that of other student pilots. But I presume that flying schools won't lose a paying customer without a fight, and it's not in their interests to be other than encouraging.
I guess the question I need to answer is impossible to answer: how much is it going to cost me to get to test-passing standard? I decided, on no very exact grounds, that I was prepared to spend 6,000 pounds on training and the equipment that goes with it. But I'm a reasonably successful, middle-aged family man, with absolutely nothing to prove, and I can easily find fun things to do with the 5,500 currently left in the kitty. I wouldn't want to spend all my money, and still not be anywhere in sight of completing the training.
What I'm wondering is if there's some way to predict how long it will take to succeed, based on a measure of progress over the first ten hours or so?