It could be an unsupported pipe somewhere else in the house. If you turn the tap off slowly and it doesn't bang then you need to find where it is doing it. water hammer transmits for a good number of metres from source.
Remove water from pipe, get a 22mm coupling, using a junior hacksaw cut the end of fitting down 15mm in length half way across its diameter, then cut just less then half of that 15mm piece off, flux the pipe and the inside curve of fitting and then solder the half moon piece of fitting onto the...
It is the three port valve that needs replacing, It may only need a new head, which means no plumbing, but it would be good if you afford to change the whole valve to a new 3 port Honeywell valve IMO they are the best motorised valves, and easiest to maintain.
On the first floor it would be easier to run the pipe work under the floor through joists with holes drilled for the plastic pipe. On the ground floor I would not choose plastic since it can be chewed by mice, It does happen. If using copper then notch but fit pipe clips to a piece of batten...
I have had a private supply like this in the past, although ours was high enough that it didn't need a pump. When our tank stopped filling I turned the water off removed the ball valve and it was blocked with bits of a frog. When I asked about this contamination of the water I was told that...
You need 1 1/4" waste pipe and the trap needs to be fitted the sink waste, a bottle trap would work well, then an elbow to take it down to the waste with another elbow to join to existing waste. You may need a coupling as well.
I will only comment on the recharging of the vessel. You do know that has to be done while the system is completely de pressurised by turning of water supply and draining until no pressure at all?
Phone the gas safe people and tell them what you just wrote here and the name of the gas fitter and they will make him decommission it and you should get your money back. Do it officially that is why we pay so much to be registered so that the customer always has a comeback to cover themselves...