For any landlords out there, don't ever use an agency. An ad can be put on rightmove via an online agency for £40, and all the legal info and documents are available to download from various websites. A couple of hours work at most certainly beats the £500 + VAT that I was asked to pay last time...
Don't pay it. I'm assuming that you have a 12 month Assured Shorthold Tenancy (AST)? In which case this automatically becomes a periodic tenancy whereby the landlord still has to give you 2 months notice and you still have to give 1 month notice (from the rent date).
For the OP, that charge...