The fossil is probably an ammonite, and as such it is not rare.
However, I can't tell which one (maybe Tabfish can), so there is a chance it is a rare species.
Would really need to know more about it to be able to name it: more work on the fossil to carefully expose it, more photos, info on location, etc.
The mineral smacks of galena to me, but again, one would need more info to be sure: more and sharper close-ups, determination of density, streak, and hardness.
It does look like a gastropod. I am on the Midlands, Staffordshire border and the hardcore could have been locally sourced, Much Wenlock? I have decided not to clean it up, as I will probably damage it. Thank you very much for your help.
Good ID by Thomas, at first glance it looks to be a Heteromorph ammonite with open coiling but it's a gastropod and not a cephalopod.
Is it rare? well nowadays they are hard to find and although i cannot see the condition of this specimen the surface looks like it could prep into something special if it is all there.
I have one in my collection but it is no way as good as yours, so well done and a very nice find.
Poleumita discors, (J Sewerby) Silurian, Wenlock series, Farley Quarry, Much Wenlock.
The information everyone has given me is really appreciated.
I would ideally like to have it cleaned up, is there anywhere in the West Midlands, that your aware of where I could take it?
I dont want to damage it.