Selling Old IT equipment in the UK

Does any one know a good company that will buy old IT equipment. We have a few old Netapp filers and Dell servers that we want to dispose of, but we need a company that can grantee that any data on disks will be securely erased before being sold on.

We are not so much conserned with getting much for it, but would like it to be reused rather than going for landfill.