In the UK it's best to get a 'Corgi Registered' plumbing/heating technician, they're monitored by the Association and their work is regularly inspected. I've seen TV shows where hidden cameras film nasty cowboy plumbers ripping off frail old people. A favourite trick is to pretend to replace a part and show a worn one, which they've actually fetched from their van. It's so wicked,an elderly person knows very little about plumbing, and will pay innocently just to have some heating. I hope the Universe wreaks revenge eventually, perhaps they'll face an old age shivering in a heatless house!