For anyone else in this situation who is not aware, by default an Assured Shorthold Tenancy (which you probably have if you're renting all of a self-contained dwelling) becomes a statutory periodic tenancy (i.e., usually, rolling monthly on the same terms) at the end of the initial contract period - this requires no action on the part of anyone involved so you can politely tell the agents where to stick their £100+ bill, assuming you don't mind being on a rolling monthly contract. You can of course do this next time you're due to renew even if you've paid for it before.

I've done this in four out of the five places I've rented over the years (the fifth being a special case as the landlord wanted to sell) and had no problems - although I had to argue the point with one agent who initially insisted I had no choice but to cough up. Pays to know your rights :-)

Sources: This guide is great and used to be on the UK government website:

http://www.leeds.gov.uk/docs/Tenants%27%20Tenancy%20guide.pd...

Since superseded by:

https://www.gov.uk/private-renting

which sadly doesn't seem to cover as much detail.