rent increases

Tenancy Types and Management

Hi. I have two tenants who share a flat and share the bills as well. Because they have got into a mess with their bill paying (or should I say lack of it)and the utilities concerned have started to threaten ME with court action I have agreed to take over the payments for two of the bills as from 1st June and I need to add this on to the rent via an increase. One has been there for 9 months but the other has only been there for just under 5 months. Can I increase the rent for the latter tenant now in view of the circumstances or will I have to wait until his 6 months. Any help would be greatly appreciated. Regards