Because of it's shape I think your specimen could be a water worn sea urchin.
But there looks to be a vein of mineral running through it suggesting to me it could be a water rolled nodule.
I agree with Tabfish in leaning towards a ovoid concretion that happened to span a vein.
It might be instructive to see inside it - if you fancy sawing it in half (you could try cracking it in half with a hammer, but it might break into lots of pieces, which could be a shame).
Hi. You're probably right, but it looks like it is a nodule which contains a trigonocarpus nut, which is why the ridges aren't visible. I'll post an image of a similar fossil from Birkdale tomorrow.