I can not see how following reg 26-9 is going to have you at the property for another hour or so, checking other appliances, most refers to the actual appliance you are working on, which should be allowed for anyway
When working on a appliance or servicing ,some people walk into the property and go straight to the appliance in question, carry out works and leave, if they come across another gas appliance, they will do a visual check and if any red flags are raised ,will take further, with the approval of the responsible party.
The first thing I do when I walk in a property is check/ask what other gas appliances there are and carry out a visual check, if there are any red flags or issues, these are raised before I do any work.
If becomes an issue, I can walk out and post paperwork through letter box
Likewise I always carry out a tightness test, I believe good practice and again to cover myself, if an issue is raised again, this is sorted before any other works carried out.
Do not carry out works then do other appliance visual checks or tightness checks( if and when you do) as if problem, could end up leaving with no payment
At the end of the day, getting legally technical, even if we wanted to, we have no right to go into people’s properties and physically operating their appliances without their permission, using energy that is charged to them, unless we have concern about there safe operation and this is achieved by a visual inspection (even then it is a legal gray area, some would say the visual red flags ,should be brought to the attention of the responsible party before continuing physical inspection and if denied ,paperwork raised)
If you think about it if you were expected to check every appliance in the property fully, every time some work was done on one of the appliances, which is not practical anyway, even if you wanted to, there would also be a rule/it would be acceptable, that no gas inspection would be required until a year after that work had been done, as appliances only have to be inspected fully once a year (If rented property)
As a foot note ,if you are just starting out, do not think the every person is interested in the safe operation of their appliances and will be happy you pointing out a fault or finding a fault that could save their lives while checking a appliance they have not given your permition to work on.
I have been to do some work on a boiler in the past, checked, other appliances, found old dfe fire in front room, which was now bedroom for children ,chimney blocked up to stop draft, along with ventilation to outside and still being used
When I capped off, all hell broke loss, tenants going to kill me, land lord going to sue the shirt off my back and agents not using me again because I am a trouble maker
So do checks and make sure done first as said, otherwise I would have put £100 worth of parts in the boiler and do not think I would have been paid somehow
🙁