how do o know which plumber to hire for big jobs?

look for nvq 3 or advanced craft plumbers work,ask how long they have been working after finishing training,unvented systems require g3 qualification,gas work requires gas safe qualification, check references and insurance details,charted institute of plumbing and heating engineers can give you phone numbers of registered plumbers.then there are qualifications for solar thermal hot water systems,and water board regulations.

Check references, customers feedback and if possible or evidence if previous work, ask how long they have been trading since qualifying and to supply evidence of said qualification and relevant insurance cover, hope this helps .