Canon GP401 Glossy Photo Paper A4 (20 + 20 free Sheets)
20 sheets goes for over £10 on most sites so this seems a bargain to me.
Make sure you get the correct one (linked) with 20 free sheets - they are selling the standard 20 pack on it's own for £6.36!
Cheapest delivery is £4.46 (free for orders over £50). I ordered two packs so that's 80 sheets for £12.66 delivered.