I levelled out the ceiling using a laser level and new plasterboards. It was more or less flat and level against a straight edge. Had the whole room walls ceiling skimmed few days ago by plasterer. Obviously don't expect walls to be flat or level as it's an old Victorian property. But when I now put flat 6 foot spirit level against ceiling, yes its mostly flat but I can see some hollow spots about 1mm. Is this to be expected since skimming is done freehand so there's bound to be variations? I'm probably over thinking this as it will all look nice and flat once painted white and plasterer seems to have done decent job overall.