In addition to their credentials and any referrals, you should always pick a plumber who can provide:

• A physical address
  
• A contact telephone number
  
• An established time in business
  
• Proof of insurance
  
• Their WaterSafe Registration
  
• Their Gas Safe Registration (if they work on gas systems)
  
• Their Competent Person Scheme Registration - if applicable. This means they are qualified to carry out certain types of work (such as installing a new heating system or boiler) in accordance with Building Regulations.