A cease and desist letter is basically where you notify the other party in writng of the dicovered infringement and ask them to remove all infringing materials withing a certain time (i.e. 14 days) and threaten legal actio if they do not comply.
In this case, it sounds like you want payment, so I would take this to a solicitor. That will also show that you mean business.
Your local Citizens Advice Bureau / Business Advice Centre should be able to recommend a solicitor locally, and he/she will be able to advise you on the best action to take.
Normally this would start with a solictor's letter stating your demands, and if not met escalate to an injunction and/or county court hearing.
Don't stress though - once they see you mean business they will probably want to come to an amicable agreement.
mmmm... I wonder if the local newspaper would consider this newsworthy? That normally 'focuses' the mind.
