i have what looks like brickdust inside sills in kitchen window.

I am reading between the lines here, but I suspect your house is pre-WW2 as modern high-fired clay bricks don't tend to produce dust. I have frequently seen this where a house built with low-fired soft clay bricks has been rendered or re-pointed with Portland cement. When they were originally constructed, they were built using lime mortar, which is softer than the bricks and forms the 'sacrificial' element of the wall. If that soft mortar, is replaced (or covered) by the brittle and hard Portland cement, as the wall expands and contracts with heat and cold, the Portland stays put and effectively 'files down' the bricks. That is probably where the dust is coming from. I have not heard of masonry bees digging into brickwork, so they are probably not the cause. It can be rectified, of course, but it's not a quick and easy fix so it could be expensive. Hope that helps. Kevin Limeclad.