If you have boiler cover or home emergency cover, your provider should always be your first port of call if there’s a problem. Most providers have a 24-hour helpline for emergencies, and you may also be able to book an engineer online.
If you don’t have boiler cover, you’ll need to look for a suitably qualified tradesperson in your area.
With a gas boiler, you should check that the engineer is Gas Safe-registered and qualified to carry out boiler repairs. Alternatively, you can contact the boiler manufacturer.