Asda (Colchester) have loads of their own brand printer cartridges at 2 for £8. I only looked for Canon PG40 and CL41 for my Pixma MP460.
The Asda colour ones are usually £13.97 and the black are usually £9.97. Both are 2 for £8. The lad I spoke to in Asda was a shocked as me!
Heres the kicker - The Canon branded CL41 (colour) has 3*4ml in it. The Asda one states 3*8ml of ink in it! So double the ink for less than 20% of the price! Bargin.
There were plenty of other carts on the same deal for Lexmark, HP etc...