The 100A fuse in the service head (or whatever size it is) does not protect the DNO cable entering your property, this is protected by the next DNO fuse upstream wether it be a 315A fuse I don't know. Only they can answer that.
Might have to argue a bit with you on that one spark ( ). The DNO's LV cables in the path are usually of larger CSA than the spur off to the property. The fuse at the s/s protects the larger cable, but the fuse in the property protects the cable from the joint in the ground (overload only of course) even though it is further along from the change in CSA.