Size Up clothing, have got mens shirts upto 5XL for £1.60 !!
Some CAT checked shirts to for £4.00 down from £35 only 1XL and 2XL though.
also, womens clothes http://www.sizeup.co.uk/MensProducts.aspx
couldn't find them on topcashback sadly.
Hope that helps someone.
Edit (01.10.2010) - GET ANOTHER 10% OFF, ON TOP OF THESE PRICES! Just sign up to their newsletter.
"Sign up to our broadcast and we will send you a 10% voucher off any of our products!"
Note - Thx Sassyblue @ MSE for this