Leaf Cutting Bee Cocoons… or so I think??

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:

http://uk.answers.yahoo.com/question/index?qid=20080710153242AAXU4Np
http://www.turning-earth.co.uk/leaf_cutter_bee.htm

I gently placed them back inside a pot of compost in the hope that they will eventually hatch.