Westminster, the tenant does not seem to be on a fixed term tenancy so section 6 is not applicable.
MrSmith, please disregard the following advice if Westminster comes back to clarify why you can't use section 8.
Serve a section 8 notice ASAP, there are details of the process here but you will need to serve the notice by hand with a witness. This is a handy witness statement (but you will need to re-register to view it).
Complete the section 8 notice under grounds 8, 10 and 11.
If the tenant has not got their unpaid rent to under 2 months worth within a fortnight, then commence a possession claim at www.possessionclaim.gov.uk. This will require a fee of £100.
If the tenant still has 2 months owing at the time of the hearing then you will get a possession order and an order for the missing rent (all of it!)
If the tenant doesn't leave whe ordered by the court, you may have to organise bailiffs (£110).
If the tenant doesn't pay when ordered to, you may have to take enforcement action: http://www.guardian.co.uk/money/2010...force-judgment
Did the tenant pay any kind of deposit (money or otherwise) to you?
If you like the information/help provided - Gizza Job
(People under my age won't understand the significance of those last 2 words)