Do you mean sections 8 & 21 of the housing Act 1988, in your supplementary information?
If you do mean these sections and not section 23 which is concerned with committee functions, my advice would be to cut your losses and get a new tenant. There are bad landlords and there are bad tenants if you have the latter you will be far better served by finding a new tenant who pays the rent. To gain possession you must ensure the section 8 or 21 notice is properly prepared and worded.
If possession is the most important I would use a section 21 notice provided the fixed period of the tenancy has expired, if the fixed period has not expired and there is at least two months rent owing use section 8.