Decathlon always worth a look. There's something for £6 on the website, but it's always cheaper in the stores - got some brilliant long-sleeved base layers there for £3.50 last year.
Best I could find in Decathlon Stockport at the weekend was long-sleeved base layers for 8 or 9 quid. Additional bonus was that they had them in colours other than dark blue! They were in the running section of the shop.
