GANZ is for the big players - GES (Gas Equipment Suppliers) belong, the energy suppliers e.g. Contact, Nova, Genesis etc all belong. Imagine sitting at this table and telling Genesis for example that they charge us too much for gas and that you aren't going to pay it and they need to drop it down. Like anyone would listen to you.
They cannot dictate how much you charge for a GSC or COC - there is a large administration component involved, along with the responsibility taken. We've told the companies we are service agents for that we will be passing on the cost and so far no complaints. What I have done is give them a different price to the public in view of volume of work generated. Perhaps if they don't like it they may like to voice their opinions to Government and Energy Safety who came up with the scheme in the first place.
The risk you run is that they remove a service agency from you.
GANZ have no teeth, they have no regulatory authority and when they aren't in the room with each other they are stabbing each other in the back. Threatening them is like hitting you with a marshmellow.
I don't agree that it should be included in your hourly rate, it is a specific one off cost per job - simple. Tell them it's like your version of the Electricity Levy that a lot of their GANZ members will be charging the general public.