S21 issued &now in rent arrears

Tenancy Types and Management

We issued a S21 4 A notice to a tenant (by delivering it by hand to the property), and it runs out 28.2.07. He has just written me a note to say he hasn't been able to pay February's rent in full, and doesn't say how/if he will make up the shortfall. He also doesn't mention leaving the property. We wanted possession as we need to do some building works on the flat above and need the ceiling down, but also he has caused us some problems and this isn't the first time he has struggled to pay on time. What do we do next? I'm also a bit worried as I don't have a signature from him to state that he received the S21, although I'm sure he has, and will this cause me problems in court (if it gets to that stage)?

Many thanks