Water softener related hydraulic shock

[Sarafailsbetter]

About 9 months now, started suffering intermittent but frequent hydraulic shock thwacks, sometimes very violent, below Harvey's water softener in mains cold copper pipe. So far, water tank ballcock replaced, kitchen mixer tap replaced, Harvey's serviced the water softener, approx. 5 years old. Nothing has stopped this thwacking noise. It never happens when washing machine / shower / toilet used, nor taps open. Harvey's claimed never to have heard of problem before and charged us even though thwacking stops when softener bypassed. Water pressure coming in is not high in inner London. Would an arrestor solve the problem, and where should it be situated? Any help / advice hugely appreciated. Sara