There is some legal issues on both sides here.
When you installed the boiler, if you do not immediately commision it, then you are required by the GSIUR's too disconnect the gas appliance until such commisioning is completed.
The customer has issue with the radiator not working, but they must give you opportunity to rectify the work.
The customer entered a contract with you for the work and has acceptance to this in so much that he has allowed the work to progress. The contract is not complete until all necessary work has been completed including the commisioning and certification. If the customer has refused to allow you to complete the contract, they must demonstrate that they have due cause to prevent this.
I agree with Tara, that the best way forward if possible, is to reach a mutual agreement. I would make sure that every contact you have is documented preferably in letters, so that if this has to go to court you can show you acted dilligently fairly.