Thanks all. Well, for the past two days he has been a different dog altogether!
The only thing I have changed is that I am using a double ended leash with his harness, and he is so much better walking with it than any other leash I've tried. No refusing to walk since I started using it - hurrah!
Apart from that, he has been more obedient generally, and even people in my village have commented on how much better he's doing.
He's even sleeping better!
I have absolutely no idea why things have changed, or why things were so bad at the weekend, for that matter. It's a mystery!
He does not have any treats to speak of. He gets his Nutriment, and I might give him a pizzle or a chewdles biscuit, or a bone if I want to distract him for a bit, and cocktail sausages cut into pieces for training treats - other than those, he has no 'human' food whatsoever, and I intend it to remain that way.