rent arrears and selling property

Rent Arrears

We have some tenants in one of our properties who have been there for over 3 years now, very nice couple, however they are now on housing benefits, they have paid their rent late in the last 3/4 months, and are now in arrears, their yearly contract came to an end and we were persuaded by the local authority to give them a six month contract which is now finished, we feel enough is enough and would like to selling the property. Anyone out there have any advise, would we have to give a section 21 or 8, or just notice. We have sent an email to housing to advise.
Look forward to hearing your comments.