Was it this?
From Belfry Bulletin No 316, February 1974
"We have received the following from the Chelm's Coombe Caving Club - a branch of the National Tower Testing Station Sports and Social Club.
On the 6th of December, 1973, members of the Chelm's Coombe Caving Club broke through into an open cave passage in their dig 'Reynolds’s Rift' at the National Tower Testing Station, Cheddar.
We have so far discovered two hundred feet of passage with a vertical range of fifty feet. A hundred feet of this is stream passage.
No visits are allowed at the present time, while clearance work is done, while a report is prepared for the National Tower Testing Station. Digging will be continued by club members.
The situation as far as access is concerned will be in the full report which will follow when clearance is completed.