I have a KEW Hobby pressure washer ...... been great to date ... but
yesterday started surging.

This is significant enough to make it useless, almost equal cadence between
pressure & no pressure ... the outlet pressure gauge is also reading nothing
.... but that may be just a duff gauge.

Anybody any experience with pressure washers ?

No shortage of incoming mains (pressure or flow rate) even tried different
hoses & fittings to be 100% sure of that.

If I operate lance without unit being switched on ... water is jetting out
OK.
Plus when it 'surges' it does seem right pressure and flow at the peak of
each surge.