Unemployed Tenants

Rent Arrears

I currently have an unemployed couple renting my property although they were both employed eight months ago when the tenancy commenced.
They have applied for housing benefit but only one qualifies as the other was previously a student before taking a job in a shop. The problem started three months ago and they seem to be developing a pattern by missing a month's rent with the excuse that they have other bills to settle and then paying three quarters of the rent the following month.
They keep the property in very good order and are otherwise no trouble.
Should I issue a statement of their rent arrears together with a letter explaining that I cannot afford to extend this form of credit to them indefinitely. I don't want to make them homeless but neither do I want to give the impression that they can continue in this manner for very much longer.