Charged for 4 months even though i cancelled the free trial

Hi, could anyone advise me on who to contact in order to be refunded for 4 months worth of wrong bills? I cancelled the free trial in time and have only just noticed that they have still been taking money from my account. Now I'm being told that if I cancel I will be charged 79£ for cancellation fee!

How can I sort this out?

Thankyou