Overcharged on security contract,?

I have a small retail business with a rental and maintenace contract for my very basic alarm system with a well known company, for which I get one annual visit at a cost of c£130.

On the last visit of the service engineer a door contact was found to be not working and the engineer replaced it within the one hour allowed for his visit, for which I have now received another invoice for £148 for a minimum 1 hour call-out in addition to the annual charge.

I have payed the second invoice as this included the cost of the part fitted, but have so far refused to pay the annual charge as it is my contention that the fitters time should not be charged twice for the same visit.

On reviewing my copy of the engineers paperwork, I see that he clocked on at 9.30 to carry out routine inspection and found the faulty contact, he clocked off that job at 10.15 and clocked on a new job no. at the same time to fit the contact which he completed at 10.30.

So, although I have a maintenance contract, they don't actually do any maintenance under the contract, but any faults found are treated as a call out and charged at full call out costs even though the engineer is already on site.

The alarm system is rented from them, should I have to pay to repair faulty parts of their system?

They are now threatening legal action.

Has anyone else had similar problems? or can offer advice.

I should also say that on previous occasions when a faulty part has been replaced, I have only been charged for the part, not an addiotional hour.