John and DBP - thanks for the advice. The suggestions about foam are appreciated - I'd forgotten the name of Microtools and hadn't thought about craft foam - excellent idea.
I'll have another go with cleaning the focusing screen. What has happened there is that flakes of degraded foam have smeared into the focusing screen. This was after using a blower, camel hair brush and then lens cloth... Any suggestions on a solvent which would break down smeared flakes of degraded foam? I've tried a lens cleaner to no avail.