site 2 says:-
Words: Augustus Montague Toplady, 1776. An unsubstantiated story says the lyrics were inspired when Toplady took shelter from a storm under a rocky overhang near England's Cheddar Gorge; he reportedly wrote the words on a playing card.

Not quite true, I have seen the plaque on the rock. Unfotunately the road was widened and removed any cleft. It is nearer Burrington Combe, about 5 miles form Cheddar.
Thanks nutty.