Ok great, thanks Guys. I will find a draft, send the letter and let you know how I get on. Should I email them or am i better off posting the letter?

Hi, I joined Energie fitness (previously Fit4Less) on 5/04/2016. From looking at the terms, it appears that it had a min term period (but does not state how long) and then one I have complete the min term, I can cancel at anytime if I give them 30 days notice. In July 2019 I was made redundant and moved over 15 miles away from my gym. Due to this I cancelled my direct debit, not realising that I was meant to give them 30 days notice. I received a call 2 months later from Credit Resolution Services stating that I owed £100 for cancelling the membership. I advised them of my circumstances and the reasons I cancelled it but the woman was rude and didn't seem to care or want to hear my reasonings I refused to speak to them anymore. I hadn't heard anything from them for over a year and have just seen that they have sent an email, advising that the debt is now £200 and if they don't receive payment in the next 7 days, they will be passing the debt to Zinc Group Ltd. I'm scared that this could affect my credit rating and i'm not sure what to do. I was with the gym for over 3 years so surely any minimum contract term should have been over, I hadn't used the gym in 2 months prior to cancelling and I had a genuine reason and proof as to why I cancelled (redundancy which according to their terms is one of the reasons to cancel the agreement). So think it's unfair for them to charge me £200, just for not realising I had to give 30 days notice. Does anyone have any advise on what I can and should do?