Whenever I turn off any tap (H or C) in my house, there is an almighty bang
as the water shuts off unless I do it v-e-r-y slowly indeed. Is this known
as 'water hammer', or is that just the 'juddering' sound some folk
experience?

I've been googling about this here, but am nor sure how to cure this easily?
I don't want to close the stop cock or any in-line valves to reduce the
pressure, as it's really not that great anyway (and I have a combi feeding
the shower - the whole place has mains pressure water, no H or C tanks).

Is an 'arrestor' what I need, or is that just for juddering pipes? If this
would do the trick, does it matter whereabouts it is fitted in the system?

Thanks