That's a motorcycle event and I wonder if there was also a car event that weekend. Can't find owt about it in Motor Sport.
Croscombe is between Wells and Shepton Mallet, so east of Wells, and there are sub-20% arrows on a couple of roads there but the event would have been on a private road which I can't find on OS 183
ETA but what I can find is a hill called Hay Hill which is to the SW of Wells near Coxley Wick. No sign of steep roads up it though.