Hi Donna hope you’ve sorted your central heating problem. To my understating you advertised a job about a radiator not working. A plumber came and identified another problem on your central heating system, with the boiler. By the way there is no such thing as carbon leaking, perhaps you want to reformulate. If the plumber surveyed the job, quoted for it and you accepted his quotation then he’s liable to carry on the works as stated in the quotation, that would be his scope of works. Please note repairing/replacing a radiator and a system checking/inspection (or boiler repair) are different jobs, meaning that unless you specifically agreed upon it he would not be liable for the boiler problem.
I see some irrelevant answers about CORGI, again unless you asked about a specific breed of dogs there is no such thing for plumbers, it’s been replaced by Gas Safe Register. Anyway, there is no legal requirement for a plumber being Gas Safe registered for repairing/replacing a radiator.
Again, according to current law cash is still a legal payment method, perhaps the most widely used in the UK.
Hope this clarifies and helps, thank you.