Can my landlord limit the frequency of my overnight guests?

I have been living in an HMO (House of Multiple Occupancy) for almost two years with no problems, until recently. I am now in a relationship (past two months) and my boyfriend spends the night about 3 nights a week. My landlord has told me this is not acceptable due to his license because the house is limited to an occupancy of 5 people. There are currently four of us living here, with one spare room. I can't find my copy of the tenancy agreement I originally signed with him, and he is claiming this is one of the terms of the agreement. I need to know if a) limiting the number of overnight guests is legal; and b) if I am in a position to argue that my boyfriend spending the night a few nights a week would be considered unreasonable? Any advice hugely appreciated - thanks in advance! (P.S. I'm in the UK).