I started to repot my tomato plants and as I tipped out the compost these little fellows fell out. They are intricately formed bullet shaped cocoons with perfectly cut rose leaves to seal the ends. Inside there is a clear looking wet grub. After a quick google I found that these cocoons are most likely to belong to a Leaf Cutting Bee.
See more info here:
I gently placed them back inside a pot of compost in the hope that they will eventually hatch.