Where to fit a water arrestor ?

Suffering from water hammer I'm thinking of installing a water hammer arrestor ? I have 2 stop cocks in my house for some unknown reason. The stop cock in my kitchen only supplies cold water to kitchen sink and bathroom toilet sink and electric shower . The other stop cock comes out the ground in a rear utility room. After the stop cock it branches off one goes to my boiler which then feeds hot water to taps the other branch feeds the utility rooms toilet and sink and then to the washing machine . The stop cock in the utility room is the one that is causing the water hammer so if I turn off taps use washing machine it makes a loudish grumble but not at the source always at the end of the line which is in the bathroom somewhere .. Where would be the best place to fit a water arrester to see if it solves the problem .

Have you always had the water hammer or could be something as simple as a ball valve washer needing to be replaced?

Had it when the washing machine runs since we moved in 4yrs ago but had new bathroom fitted and seems to be all taps using hot water that causes it now, how does a falty washer cause water hammer when it only happens when turning off taps or washing machine valve closes, the taps are only if there shut quickly,

Try turning the stop cock off slightly. Or if theres iso valves on on the problem taps try turning those down. There is no real quick fix its trial n error. Is it a gas combi boiler you have?

Yes combi boiler all the taps have ISO valves could turn them down a bit tried turning the stop cock down then up etc no difference just made the pack nut leak and had to ptfe that lol

Turning down the iso valves should fix the problem. Failing that a small expansion vessel on the hot outlet about a meter away from boiler with a check valve between vessel and boiler