the lettings agent is not responsible for getting the tenant out, by the way – whether you have a case against them to do with references etc. is another thing, but they will only be able to advise you – the legal advice and the way you proceed is down to you.
but as has been said numerous times in different ways, the only way you'll get legal possession back by the book is by getting a legal expert to serve the correct notice and get an eviction hearing, so the sooner you do it the better (the date on which this can take effect is governed by the payment dates of the tenancy). so get on it!
this: http://www.landlordzone.co.uk/Legal/Residential/Tenancies/ending-assured-shorthold-tenancy.htm
may prove useful reading, to give you an idea of the position.