chauffagist

Hello,
Anyone with any advice on steps to take for a chauffagist who did a repair job but in the process seems to have damaged the boiler. Not responding to phone calls to even listen to the problem. Is there an association of chauffagists I could contact to complain about them or at least to know if they are registered - should have checked this before, I admit but took the plunge in calling them out. Any other advice welcome. Thanks.