Congratulations! I asked my colleague in the museum, who confirmed that this is indeed worked flint.
The percussion lines on the fractures showing they were formed with great force and the presence of fractures all the way around the edge are good signs for it being worked. This can be seen particularly clearly on the second picture, where you can also see that the right hand edge of the flint appears more worn than the left side, suggesting that it has been used.
It is the right sort of shape and size for a scraper and Bronze age is the best bet at the moment. Finding it in an exposed buried forest is an excellent sign.
If you tell us whereabouts you found it, this should give us a much better indication of the age and we may be able to tell you more about it.
Thank you for the reply and your interesting comments. The flint was found at Skipsea on the East Yorkshire coast. It was lying at the foot of a low cliff as if it had just fallen out. At the spot there are stumps of trees exposed, many lying horizontal in the cliff. I am told this is a buried forest which is exposed at a number of locations on the East coast.
As Luanne mentions, it's a flint scraper - a kind of multi-purpose tool that could have been used, amongst other things, to clean animal hides. It's difficult to date and I'm far from expert, but was most probably produced during the Neolithic or Bronze Age (from the photo it doesn't look like a recent bit of flint-knapping). Someone else may wish to add to this though.
You can see a lot of 'retouching' (the small jaggedy bits) all around the edge, which is a sign of sharpening or resharpening. Hence, it's quite possible that it has seen a bit of use in its time - which is a pretty cool thought!
Retouching is a really good sign that you're looking at a piece of worked flint, rather than something naturally formed.
In terms of the flint itself, then it's a type that is common within the glacial deposits (i.e. cliff) within the Skipsea area, so most likely has a local origin. There's evidence that people have lived in this area on and off since the end of the last ice age (the past 11,500 years or so), and the eroding cliffs are a great place to spot evidence of this!
Thanks for your additional comments about the flint. These are very interesting. I have since read in books that other prehistoric finds have been made in this area including barbed antler points. It is one of my ambitions to find one of these so I will keep searching. A colleague of mine told me of an amateur archaeologist who lived at Skipsea and had quite a collection of flints and other objects. I would have liked to have seen his collection but I understand he has now moved away from the area.