Littlewoods Service Renewal Guarantee

In 2014, I bought a laptop from Littlewoods; along with it, I bought a Service Guarantee. In 2016, Littlewoods renewed the Service Guarantee without my knowledge or authority.

Littlewoods' stance is that, because they automatically sent out a Service Guarantee renewal letter, they had the right to apply the renewal charge to my account. However, I did not receive the authorisation letter, therefore could not respond in either sense.

I have corresponded with Littlewoods after submitting a complaint (I have chat transcripts and telephone recordings) and although they have admitted it is possible I did not receive the letter, because of a possible postal problem, they believe they were right to assume that I had received the letter, and furthermore were right to apply the charge because it complies with their Service Guarantee renewal procedure.

Has anyone had any similar occurences with Littlewoods, or any other company, if so, how did you proceed?