There's no way you can still be "under contract"; you must have passed the min. term of 12 months years ago if you started with Freeserve. You should insist on speaking to their Retention Dept. and ask what deals they can offer you to stay with them. I've been on 8 Meg BB Plus for under £10 on 12 month contract with 3 months free for last 2 years.

Alternatively you can ask for your MAC and go elsewhere, but you won't get much better speeds as you are getting around the best for your attenuation.

Well, they have been taking £17.50 out of my account every month for years and I've never been disconnected for more than a day so I don't know what's going on.

They probably automatically roll over the contract, i.e. they'll keep charging until you tell them to stop (by completely canceling broadband or by migrating away after getting your MAC).

Yes, that's because you've never cancelled the service and never entered a new contract for a lesser amount. You were always on your initial contract but beyond the Minimum Period and so were subject to giving 1 month (or more) notice of ceasing or migration (which you never did).