Thanks for the many and good photos.
Sorry - no - it is not a turtle fossil, or any kind of fossil.
Erosion has made it smaller, so we can't see the larger structure, but it is either/both:
- fault breccia
- concretion similar to a septarian nodule.
Some septarian nodules really do make one think 'turtle' - the ones that are complete, flattened and appear to composed of interlocking plates. I came across one in the museum at Lisdoonvarna (I think) in 1977 in the west of Ireland that is labelled 'turtle', but it is just a septarian nodule. (Nodule/concretion: much the same thing.)