@ Hugor, I think that’s Somerton Combe in the top picture, if it skirts round to the left then dives down into the woods and takes a right before going all the way to the bottom then it is. One of my favourite descents on the quantocks! I can’t get an OS map on my iPod but I would have thought Frogs would be labelled on a map, I think it’s one valley to the east of Somerton, can’t be sure without checking though.
Edit: just clicked mad bill’s link and got an OS map layer, that’s stert leading into Somerton, it’s still awesome though! Frog Combe is the next Combe over from Stert, very different depending on which side of the stream you go at the top, the west is sweet, flowy singletrack but a bit of a gully right at the start which suffers in the wet, the east is a bit more technical with a few rooty drops before the two meet at a crossing and you get the traditional quantocks Combe descent, criss-crossing a stream all the way to the bottom!