It is most probably a chert nodule. If it is from East Mendip, you do get chert nodules in the limestone (particularly common in the upper part of the Black Rock Limestone), but as ever it is hard to tell from a photo, especially when caked in mud. The other possibility is an oncolite (an algal concretion akin to stromatolites) which do occur in the Clifton Down Limestone, but the morphology doesn't look right. Best to clean it and if it is very hard and siliceous, then it is chert.