Looks like my frames when the weather finally warms up and the bees go pollen mad after the east winds of death. They stuff it in any vacant cell. Could also be hygenic bees removing larvae because of disease or interbreeding.
I think the point about most beekeepers not recognising disease when they see it has been well made.
The frame is from a colony with a bad AFB infection (from Portugal not UK). It has the classic pepper pot pattern of brood. I know I always examine anything that looks like this in my hive's very carefully. To date it has just been an uneven patter of brood emerging with clean empty cells. It's when those cells have "stuff" in them it's time to worry.
Alfred, mites can easily be treated and a new swarm is an ideal situation as all the mites are phoretic and easily dealt with.