[Gas engineer’s address]
RE: Boiler [installation/ repair] on [date of boiler work]
On [date of boiler work] we contracted yourself to [install/ repair] our gas boiler.
Under the Consumer Rights Act you are required to complete the work using reasonable care and skill using parts and materials that are of satisfactory quality and fit for purpose.
We do not believe the original [installation/ repair] conforms to this for the following reasons:
We therefore we now request that you put right the work within [set appropriate deadline].