Yes this is a bone or at least part of one. It is the very back end of a skull, the part which attached to the neck. The top part of the bone in the top photo there are two symetrical curved surfaces which are the joints for the first neck vertebra, the hook shaped piece on the right of the photo is for attachment of muscles, the mastoid process. It is herbivores which have this feature, such as cow or horse. There is no indication of size so if it is small Sheep would also fit.
I cannot tell how wide it is as there is no measurement scale. If you could post another photo looking through the large hole, like in the bottom photo but from the other side and with a ruler then I can directly compare it with reference skulls.
The width is given as 14cm, a bit needs adding for the broken mastoid process, I've got cow and red deer skulls both at 15cm across the mastoid processes, other differences appear subtle and I couldn't pick out anything conclusive comparing this photo with my specemins.
Thanks for pointing the size out Richard. Unfortunately my Horse skull is out on loan but I am getting it back tomorrow so I will take a measurement and do a direct comparison. I will post back.
My horse is only 12cm across the mastoids (and it is a big horse skull at over 60cm long). Plus the central bit (I don't know what it is called) appears much narrower in comparison to the deer and cow. Will be good to have other measurements to compare with though as mine might all be unusual!
I have my horse skull back from loan, it is a female pony from the Gower, 11cm across the mastoid processes, so entirely consistent with Richards larger skull. Looking at the shape of the condyles and mastoid processes they are both quite different to the photos shown here. So I am with Richard.