Thought I'd give an update in case it helps others.
The company that bought out KEW ... Nilfisk-Alko carry no spares at all for Kew. Apart from hoses & lances, nobody seems to stock parts for Kew ....a pity as it seems a substantially built machine ... large stainless steel water-cooled motor, multi-stage pump, rugged brass castings.
In my case the 'surging' was assumed to be the unloader valve, tripping at too early a pressure.
The screw head slot visible in picture ... did 'not' adjust the spring pressure, as it turns freely ............ you would have to insert a socket at other end of shaft to hold the nyloc nut while you turned the screwdriver ......... I didn't find that out until I stripped the valve.
The very large brass nut in the picture was undone ...and it took a huge amount of torque to get it out (and a big Stilsons) this then unscrews the unloader valve.
Once out I checked spring .. all seemed OK, I physically rotated the 'poppet' valve on it's seat, it sound a bit crunchy initially then OK ... could se nothing wrong ...... so reassembled .... and it is now working fine.
SWMB is now out using patio scrubber attachment and doing all the patio & paths (she enjoys it)
So it was the valve (thnx to those who explained how it worked) ... assume something was stuck or there was crud somewhere.