"Rod" wrote

Not so in this case. The stopcock under the sink controlled flow to the
tap at the sink and the washing machine only. This is a branch from the
main water pipe (from after the whole house stopcock). (The main run of
which carries on up to the loft, the the cold water and header tanks.) It
would have been more usual, nowadays, to have a service valve of some sort
under the sink.

There was and still is a 'proper' (i.e. normally positioned) mains
stopcock for the whole house supply in the boiler cupboard, where the
actual water main pipe enters the building.

In that case it would appear that the branch line was acting as a shock
arrestor, rather like a damper, with the reduced bore of the stopcock
providing the damping effect.
As others have said, it looks like it is time to install a proprietory shock
arrestor somewhere to see if this resolves the issues.

Phil