That sounds unlikely with the size of capacitance given. Spark
suppression capacitors require a low ESR to absorb the RF and a
capacitor of 25uF is not going to have that typically. Spark
suppressors are normally in the 0.1uF range - further I doubt that
pressure washers use universal motors so there are no sparks to
suppress.

I converted a 2kw motor from a broken pressure washer to run my
circular saw some time back. In doing so I had to replace the timer
capacitor and the start/run capacitors. The timer capacitor in my case
was in an electronic circuit and was small - the start/run capacitors
were like the ones shown in the Maplin advert and were of that physical
size, voltage and tolerance. If it is a timer capacitor it is most
likely to be electrolytic and have a tolerance in the order of 50% or
more, so any of the values given will do.

The Maplin site has a website http://www.lcrcapacitors.co.uk/ - see
if that gives any guidance on the accuracy requirement of start/run
caps. As pressure washers are run for quite short periods I wouldn't
have thought that a slight change would have made much difference and I
would go for the 30uF.

Rob