Further to post of Pre-owned XBOX360 20GB HD for £9.95, I went to my local Blockbuster (Tamworth) last night to pick one up, which I did for £8.96 as I had a 10% off voucher, but also noticed they had the 60GB for £14.95 and the 120GB for £19.95. I only needed the 20GB for game saves so I was happy to have that. They had about 4 of each left, I assume it's nationwide.
Hope it helps someone.
Note, instore only, link just takes you to the Blockbuster website.