calculation of 2 months rent arrears

Tenancy Types and Management

We have a tenant that generally pays part cash and part cheque - he has only been there a few of months. Usually so far the cheque bounces and he rushes round with extra cash. We have just recieved our November rent in this form (almost a month overdue) and the December rent is now due - we were all set to serve a section 8 notice. The rent is 690 a month and 440 of the November rent was paid by cheque. If this cheque bounces the tenant will be 440+690 behind. The question is how do we calculate the two months overdue rent we need for a section 8 notice if some of it is not full months?

Do we have to "credit" him with a a full month when only a part month has been paid, or can we issue the section 8 notice on a pro-rata calculation through the month?

We suspect this guy is pro.

Thanks, David