Avoid at all costs, cowboy plumbers are mainly multiskilled personnal, no formal training in Plumbing, no knowledge of the water by-laws and no insurance [ public liability ] and normally charge the earth for their services. Also avoid the larger Companies because they can sub-contract their work out to so called Plumbers, and by doing this increases the cost of the job. Plus they can charge VAT which really bumps up your bill. So even if you have a Plumbing emergency try and stay local to your area and make sure they are registered with the Institute of Plumbers based in Hornchurch Essex.
Or better still use someone who comes recommended.