Agree with 4getmenot - Get all copy correspondence - Also have a look @ S.40 Administration of Justice Act 1970 re: causing harassment, alarm or distress in pusuing what is claimed to be a debt.
Debt Recovery Agencies are not Court appointed bodies. Their representatives are not Court appointed bailiffs.
Read S.40 of The Act and respond once you have received all copy correspondence. - Point out the offence they are comitting if they continue to harass you in this manner.