The Medway Handyman wrote:

Is the fixed inlet pipe rigid or flexible? Prossibly caused by the by pass
valve allowing pressurised water back into the inlet. Stop connectors are
bad news on pressure washers.

It's rigid, here's a pic of the old one:
http://www.willcoxonline.com/splitpipe.jpg

Thinking more about it, I'm not sure how the stop valve could cause a
problem, as it is physically held open by the male part of the
connector, so extra pressure on the inlet side should not be able to
force the valve closed?
However, not wanting to risk it again, after fitting the replacement
part today I used a plastic, non-stop valve on the end of the hosepipe
and it is working fine now!

Ben.