Edinburgh Meadowbank TKMaxx has them for 20 quid, mostly in size 7 but some smaller sizes.
Always go down a size in Birkies. I'm a 6 or 6.5 usually, but wear size 5 Birkenstocks.
Everywhere else has them for £35, but you can get the patent ones on ebay for £26 including postage if you hunt about a bit.