As we stared to take it apart we found one skull, some vertebrae and a couple of tiny ribs. It smelt quite unplesant.
As we dissected it more and more we discovered that there was two skulls and two tails (you can see one tail in the bottom right hand side of the photo below).
Once we had taken most of the fur away we could identify shoulder blades, hip bones and the lower jaw bone.
I put this photo on Twitter and asked if anyone knew what these skulls belonged to, it was suggested maybe rats or vole.
A friend sent me a fantastic owl pellet ID guide where you follow something like a flow chart. I think the big eye sockets and the hip bones make it a rats skeleton.