Nice to come across another Derwent AA member… Have you had a look at the
Derwent since the weekend?
I know that Angel of the North has evening carp on the fly sessions, but it's not terribly cheap and probably not much use in the winter either. (It's also not exactly everyone's cup of tea either, mine included.)
Apart from that, I can't think of many other stillwaters that allow fly fishing. Thornborough perhaps? I've certainly never seen anyone trying, but then again I've hardly ever seen anyone else down there. Four in One membership's a great deal anyway, I'd say.
As well as the Durham free stretch, don't forget that Derwent AA's got an exchange ticket deal with Ferryhill so you can have a bash on their bit of the Wear too. They've got grayling (not as many as the Derwent though) and a fair head of decent chub and dace too.