I'm struggling to get the pressure washer switch out, work out how it
fixed - which seems to have failer o/c. I wonder if anyone has had to
replace one before and found out how to release them?

I have seen an exploded diagram, but it doesn't give many clues away.It
does not seem to be called a pressure switch, rather a 'release valve',
but its the item which switches off the pump when you release the
trigger on the lance/ prevents the pump building up too much pressure.