I agree with John (mm) those letters are the only proof that someone is using your identity.
I would write to the companies involved and inform them that you are not the people they are after but your address is correct and tell them how long you have lived there, keep a copies of all correspondence and send any letters by recorded delivery.
Like the telephone preference service you can get a similar service from the Royal Mail.
Before you recycle your paper remove any traces of your name, address an identifying codes on all correspondence.
Never fill in or answer any surveys, offers of prizes always come with catches. When you fill in guarantees and the like only fill in the sections of your name and address and the item you bought, nothing else is relevant and you can bet that these details will be sold on.
If it continues then you must report it to the Police if only for the incident number, this again is proof of what is happening.
Regards
Alan