Gas Safe have forms for this. Its usually a 3 x duplicate Service copy form ( White/Pink/Yellow ) 1 copy for customer 1 for landlord if needed and 1 to be retained by service engineer. Flue analysis is mandatory now for boilers / grills / ovens / fires and you should have a copy of these with the record if there not cowboys. the form can include up to 4 appliances. they should also rate each appliance to make sure correct pipe sizing to each appliance to confirm gas supply is adequate . a bit like cable sizing with electrics.