Pronet Refused to Remove Their Equipment After My Contract Ended and Now Charges Me

I registered with Pronet for security services, and while our contract expired, they never came to remove their equipment despite several enquiries. They first informed me I could keep the system free for three months and that should I still want it later, they would collect it. It has now been six months, and instead of taking the equipment off-site, they are requesting money. Their lawyers are contacting me even though I have a fresh system set up. All of the at least eight different customer personnel I have interacted with promise that the problem will be fixed, yet nothing has changed. Every discussion I have is recorded, hence Pronet should take quick care of this.