I found these amongst stones used to fill the gaps between sleepers on a minature railway line in Kent. Nos.007,013 & 015 are set in a flint-like stone and I would like to know exactly what, and in particular how old they are likely to be. 013 & 015 is unusual and has parallel stripes on the underside and is more of a horseshoe shape. Thanks for any info.
Your right, 007,009, and 008 are all flint enchinoids (sea urchins). As for their age, all flint forms in Chalk so would be the same age as the flint bearing chalk in the UK which is Late Cretaceous (90-70 MYA).