If you click the above links they will take you to a couple of pages that both show a considerable number of Dinosaur Egg pictures.
If you search the web for Dinosaur Eggs there are a lot of very good sites which show pictures of eggs that are very much like yours.
I'm no expert, I just have an interest in fossils & rocks. However, for what its worth I think you have a Dinosaur Egg. I'm sure someone else with far more knowledge will confirm whether it is or not.
It looks like a septarian nodule to me. These form in sedimentary rocks as balls of mud/clay that crack as they dry out and then the cracks are often later filled by calcite or other minerals. They often form spherical shapes like yours.
They can be pretty large such as these in New Zealand - http://en.wikipedia.org/wiki/Moeraki_Boulders
We dont get the giant ones but otherwise they are found over from dorset to yorkshire that I know of and probably anywhere with sedimentary rock exposed.
They are often broken open by fossil hunters as the centre can often hold ammonites or other fossils that the clay built up around.
I wanted to be absolutely sure so I showed your images to some specialists in the palaeontology department who cinfirmed that this is not a dinosaur egg but it is a concretion. The outside of the concretion has cracked in the way of septarian concretions. I've attached a sheet below that explains what these things are.
Hope this helps,