Legal point

Tenancy Types and Management

I have a tenant I wish to remove. I am in a position to issue a section 8 but I just need to clarify an issue.
The tenant was a friend of my partner and moved into the property before it was really ready for letting. The monthly rental on the contract is £640. We agreed verbally however, as the property was still in need of decoration, that I would only charge £575. The tenant fell into arrears and so the decoration was never completed (to act as an incentive). The tenant has now fallen heavily into arrears but what do I do with the figures on the section 8?