Holding back rent payment

Tenancy Types and Management

I'm looking for some advice if anyone has been in a similar position:
We have a tenant who signed their agreement on 28/04/06, but say that because there were problems with the flat, they didn't move in until 23/05/06 and are refusing to pay rent for this period. I have checked with the agent and they have said that they checked the flat and it was fine, and although there may have been some samll problems with the flat, it was certainly nothing that would stop them from moving in. We have offered to split the difference with the tenant as a compromise, but he has flatly refused to pay. All rent since this date has been paid, but effectively we are always a month in arears. Any advice on what to do would be great - we seem to have hit a brick wall!