A plumber
Plumbers commonly repair water pipes, taps, toilets, basins, baths, showers, radiators, tanks and water-side heating components. They may also install bathroom fittings and alter water supplies and wastes inside the property.
A drainage specialist
A drainage specialist may be the better route for a blocked or damaged drain, repeated external blockages, drain surveys, root ingress or faults in underground waste pipes. If only one sink, basin or toilet is affected, an internal plumbing blockage may still be the likely starting point.
Responsibility for drains and sewers can change at the property boundary or where pipework is shared. Your water and sewerage company can explain the arrangement in your area.
A Gas Safe registered engineer
Work on a gas boiler, gas fire, cooker, gas pipe or other gas fitting must be carried out by a Gas Safe registered business and an engineer qualified for that category. Some plumbers are Gas Safe registered; others are not. Check rather than assume.
Water-side radiator and pipework jobs may not be gas work, but any job that involves opening, repairing or connecting a gas appliance needs the appropriate registration.
Gas or carbon-monoxide emergency
If you smell gas, suspect a gas leak or are concerned about carbon monoxide, call the National Gas Emergency Service immediately on 0800 111 999. Open doors and windows, avoid flames and do not operate electrical switches. Move into fresh air if carbon monoxide may be present; call 999 or 112 for severe symptoms or collapse.
Sources and further information
External guidance can change. Follow the linked organisation where its advice applies to your situation.