Hi Rob, you may find this link -
P-05: Copyright Infringement - Information fact sheet
to the UK Copyright Service's page on copyright infringements useful. It sets out step by step what you should do in these circumstances.
Here's another useful page on enforcing copyright from the Intellectual Property Office's website -
http://www.ipo.gov.uk/types/copy/c-m.../c-enforce.htm
Good Luck
Jeff
(Schedule 1 Licence holder for Kingfishers, Barn Owls & Avocets)