Thanks for everyones response. Definitely drone cells. Went out yesterday and tried to take photo's, but, my camera has spent a year in my glovebox and I think the heat has made every pic a strange green colour! So no go.
Also read today that lots of drone cells, especially in burr comb is a sign of swarming soon? Anyone with experience on this? The bees are working great though, packed on probably 25kg of honey in a week, although still a bit rough for me!