Earth bonding gas and water

6mm cable is the wrong size to bond gas and water & it Should be part of consumer installation job.

Gas/water equipotential bonding should be 10mm as to the charges this depends on the installers method of pricing up i.e. does he price separately for different parts of the job or an all in price as I do but its good practice to make these costs known up front and not add them on at the end just to bump up the profit. Are your electricians actually registered you can check on the electric safe register on line as all registered electricians are on it. Did you receive a full installation certificate and have you received a compliance certificate in the post within a month of completion(usually within 10 days).