Thanks everyone, I really appreciate the advice. I will get a good enzyme cleaner; I have been cleaning with normal cleaners only. I've barricaded the dining area (quite a complex endeavour!) and so far... Touch wood... He hasn't gone near there, nor has he peed indoors at all. Mind you, I don't think that is him being clever so much as us just trying to keep him outside for as much as we could manage today! - I took to the backyard with a comfortable chair and a good book. I did succeed in catching him doing the deed once, but mostly I have to think he did it without my help (you have to be quick to spot him!).
We leave the back door open as much as is possible; he doesn't have to wait to go outside.
Yes, I got him from a breeder, but I don't know what she did with toilet training.
He does have an indoor crate. Unlike my older dog - who walked into the crate and claimed it as her own on the day we brought her home - my puppy has shown no sign of taking very kindly to the crate. He has gone in there voluntarily once!
Mostly I carry the puppy out if I think he wants to pee, cause he's not so fond of outside! Although today may have improved that - the older dog hadn't taken to him too kindly before today - nothing "bad", she mostly just ignores him (and has been very quiet) - but today they ran around together a lot and my older dog was back to being her active happy self.
Right now I'm sitting outside waiting :) someone had a big drink half an hour ago and there's no way he's going in that crate without some action first :)