Thanks for your replies. I was worried about ticks too, but I can't find one. He's licking around the base of his tail a bit which makes me think it may be a bit localised. He can wag it a bit, but he can't pick it up very far. I would take him straight down to the vet but DH went past there on his way to work and knocked on the door - there's no one there I've now been told to call back at 8, and someone should be there. So I reckon I'll just be rocking up to make them see us
Definitely see your vet, insist that she be seen. My old doggie had a similar problem once before, turns out she had injured her tail (it's a set of bones that continues like an extension of the backbone) and she got quite lethargic and sick. Apparently when they are developing an infection it can affect the way their hearts beat (??)... I don't quite understand how it affected her that way but she needed a shot of ABs and some high-powered anti-inflammatories to sort it out.

ETA - just saw your last post - sounds like a similar thing - injury to the tail area.