I was just moving a couple of locally-collected crab apple plants between growing cells and found these odd capsules made of leafy material. I took one apart and found a round ball of what appeared to be chewed up woody material. Not having seen anything like this before, I did a bit of online research and found it was produced by a leaf cutter bee. The egg is laid in the round woody ball. There are a dozen or so species of leaf cutter bee in the UK. Apparently they are not a particular problem for trees.
More info on bees is on the RHS website
