AFAIK, there is no single comprehensive register. How it works is as follows:
The plumber takes a 'G3' course with an accredited trainer who issues a certificate and photo ID card giving details of the dates the qualification is valid for. Just ask the plumber you are considering employing if they are G3 qualified and say you need to to see their card before the work starts.
Some 'competent persons' schemes, e.g. Gas Safe, allow the plumber to add their G3 qualification to their scheme register so the qualifications can be listed on one card and the paperwork is streamlined for the member. You can search the Gas Safe register online but not all plumbers that done have G3 are Gas Safe registered.
If you want to find a G3 registered plumber and are having difficulty, the cylinder manufactuIv rer will normally be able to give you a list of possibilities in your area. Consult their website.
Is this for you to join?
Is this for you to join?
Is this for you to join?