Whilst LensPens and SensorPens work brilliantly on lenses, sensors and mirrors of DSLRs, do not attempt to use one on the focussing screen (the translucent white area above the mirror with the AF point markings on it). The minute particles of cleaning compound will remove larger items of dirt, but in the process they are small enough to get lodged in the fine fresnel markings etched into the screen, resulting in an almost soot like coating.

So now, instead of a screen with some small smudges on it my neighbour has a darkened screen with very subdued AF point illuminations. Despite not being an officially replaceable part on the Canon EOS 550D, it does appear to be fairly easily available, so hopefully I’ll be able to rectify my mistake properly later this week.