Dat
Check out Fibrax products at

http://www.fibrax.co.uk/

They make replacement pads for Shimano/Campy/Magura cartridges, as well as their own.
They are as good, if not better than any other brand, and cost a lot less, and any UK bike shop will stock them. I use them , and can recommend them.
Shimano pads stop well, but are notorious for eating through rims.