Jesus Christ!!!! Some of you have a pretty wierd idea of the law.... No wonder we have a bad reputation and face ever increasing regulation....
On the face of it, the customer has asked for a combi to be fitted by a local plumber.
Since fitting a combi involves tinkering with gas, then it is for the plumber to say if he can work on gas or not, if he holds the realavent corgi tickets, then this part of the arrangement is fine... If the plumber informed the customer that he is not CORGI regd and the customer said "I don't care... just get it fitted" then the problem still lies with the plumber as he is the one breaking the law... ( If your boss sent you out in an untaxed van with bald tyres, then it is down to you to refuse to drive it and it is you who will be nailed if you are caught)
With very few exceptions a band A or B boiler must be fitted. Just how the boiler manufacturer achieves this is down to them, but generally speaking a Band A or B boiler is a condensing boiler.... It is down to the installer to make sure that that the boiler is suitable for the job... If the installer has quoted for a boiler then it must comply with current regulations... He is the expert, the professional, the guy who the customer relies on to advise... It is not down to the customer to constantly check the installer is doing the job right.... If you bought a brand new van, wouldn't you be upset if the police pulled you over a hundred yards down the road and booked you for not having lights and seatbelts fitted?....Especially if you went back to the dealer to complain and their respose was "Well you never asked for them, mate it's down to you to check that it complies with the construction and use regs"
If an installer charges to do a job, then at the very least it must comply with regs and the equipment must be what the customer has been quoted for, and will ultimately pay for... If a non condensing boiler does not comply, the the assumption is that a condensing boiler will be fitted otherwise the selected boiler will not be fit for the purpose