[chris (OP)]

This was found in south Cambridgeshire, UK. It looks like a stone that's been attacked by the flint equivalent of woodworm! There are signs of these pits running right through the specimen. The images below are the "top" and "bottom" surfaces of the piece.

Can anyone offer a diagnosis for me?

FOSSIL-1.jpg (803.46 kB . 2476x2724 - viewed 6324 times)
FOSSIL-2.jpg (847.95 kB . 2458x2698 - viewed 6450 times)

[Bill S]

Rough guess.  The holes were there first, in the form of borings possibly with a CaCO3 coating. The flint nodule was precipitated around them from a solution rich in silica.