This fossil looks like half a ridged cylinder sitting on top of a roughly triangular base. The half-cylinder is slightly skewed in profile along its length and each end is rounded. Any help much appreciated!
I am pretty sure this is a pseudofossil - that is, something that looks like it ought to be a fossil, but is not.
It is made of flint, and in this case it is (partly) banded flint.
Banded flints really can look very organic, but they are mineral.
Have a look at these:
If you do a Google image search for 'banded flint', you'll find a lot more.