Shhh, 1and1 don’t want you to know this, but…
… you can cancel or amend contracts at
contract.1and1.co.uk
As far as I can tell, this information is not to be found on the admin.1and1.co.uk FAQ or Help pages. I had to complete an online support request to find this info.
Once you are in, it’s easy – so I have started the process of releasing four unwanted domain names.

Update 2nd December 2010: the URL has changed to cancel.1and1.co.uk – still not well documented.