pg8uk: I suppose it is possible that somebody has given them your details in order to avoid payment themselves. Do the communications mention you by name, or is the correspondence addressed "To the Computer Owner" (or a similar form of words)?
I don't know how up to date you are with developments, but you may want to check
this out as it will perhaps lay your fears to rest. You may also want to contact Michael Pollitt as he is interested in cases like yours. You can read more about his interest in the matter
here where he also gives details of how to contact him.
If you want further information, please feel free to email me using the facility on this site. And we'll look forward to hearing how you get on with this.