Hello Robert, I'm afraid I don't know the area, and without it 'in the hand' it's going to be more Q's & A's! you say it doesn't react to acid, but it is 'heavy for it's size' This would say 'flint' to me - will it scratch with a steel object? What is the path made of (bedrock, gravel, combination of....)? I'm afraid your new photos are not suitable to enlarge to see detail, and I don't see the indentation of an 'evolved' heart urchin. It does look to be similar to a Nucleolites sp.
but this is a Jurassic genus and would not be in flint.
yes it does give fine whitish powder when scraped which Does give off CO2 in acid.
I found the echinoid on top of the Birdsall Calcareous grit bed actually on the Brigg which is of course Jurassic (Oxfordian) and very hard!