I'm no expert but I dont think so from the photo's.
First thing to think is would this have made a useful tool - does it sit in the hand nicely with a blade or spike in the right place?
Next does it look worked - does it have evidence of many impacts forming a nice edge or point? The less impacts the more likely it is just to be natural and you have to be wary because a hard frost like we had last winter can break off small chunks to give a rough impression of a worked piece.
The other type of thing to look for are cores - these aren't tools of themselves but have had blades chipped off of them.
If you find a really convincing one you could contact a local expert through the Portable Antiquities Scheme - http://finds.org.uk/
I had it checked out by an expert in ancient humans just to be sure and he confirmed that sadly, no it is not worked by man, it is just a naturally broken piece of flint.
Better luck next time!