I think you have found a very nice ammonite and if you look closer you will probably find some more.
Quantox Head sounds to be in your area, it is a fantastic place to find fossils especially big ammonites.
Its a bit of a way from quantox heads, I found it at marsland near hartland. I have spent quite a lot of time looking at the rocks and geology there but have never found anything like this in this exact location. I'll keep looking and see if anything else turns up!
Has it migrated from a collector's garden? I've found wonderful specimens on people's rockeries! (and come to think of it, left specimens on rockeries when I was younger and didn't know better!) Rx