Graham Tattersall drew him, partly inspired by a previous character, who might have been called Oggy.
Also mainly inspired by The Slag Brothers.https://tenor.com/view/the-slag-brothers-rock-and-gravel-hanna-barbera-caveman-gif-14998308
Some quotes from Graham:
"If I recall correctly, I bastardised a previous drawing that was in all the caving club papers. Captain Caveman was certainly an influence"
(see picture below of old t-shirt)
"Yep, that was it. I added the beer but can’t claim credit for the club.
I had a blue version of that T shirt when I first joined. Might have even had a yellow one too?"
"Oh, and I have no idea who called him Stig. I thought that was Andy, or maybe Snowy? Who knows?"
Andy's reminiscences on the name:
"I do not remember calling him Stig, but it was a very long time agon now and my memory was not great to begin with..."
"I also heard him referred to as Oggy. This was the back of the previous sweater which I retired to the bin last year after 30 years of service. Looks like the beer was Graham's addition?"