Low service standards

I have sold my 5.5-year-old vehicle. On cancellation of the Netstar contract I have been told that the contract still has a number of months to run and that a penalty is payable, I had a Netstar tracking device installed when I purchased the vehicle in 2017. This unit malfunctioned and I requested Netstar to replace it. If it had note malfunctioned there would have been no need to contact Netstar. This replacement unit is now being regarded as a new contract by Netstar. I was under the impression that the contract is on a month-to-month basis. I have requested that proof that I have agreed to the new terms and cancellation clause be provided to me. Numerous e-mails have gone unanswered, I feel that the attitude of Netstar is totally unacceptable Netstar did, without providing any invoice or basis of calculation deducted the so called amount due from my bank account,