It is part of a sea urchin (echinoid), preserved in flint.
The round bits are where the spines attached (they are like ball and socket joints to allow the spines to move).
Here's a modern one with prominent spines - http://www.flickr.com/photos/barryfackler/7143078355/.
Here's an example like yours, that shows yours' features better - http://www.flickr.com/photos/the_fossil_store/5384207529/
Maybe somebody with local knowlege can identify the genus (we have some rock hounds watching these postings)...