It's certainly not mentioned in John Forder's 2001 list of fatalities in the Dales (1936-2000) - which was compiled much closer to the time.
However, the April 1988 Unnamed Cave (in wikipedia) is said to be at Stone Rigg (in above list) - the only cave in that locality seeming to have a sump is Lower Stone Rigg Cave, although the guidebook doesn't say when it was discovered.
Also, in Life and Death Underground, Lovelock ascribes 11 deaths to Alderley Edge (whih would be before the latest one in Wikipedia), & would take it ahead of Porth yr Ogof...
However, the April 1988 Unnamed Cave (in wikipedia) is said to be at Stone Rigg (in above list) - the only cave in that locality seeming to have a sump is Lower Stone Rigg Cave, although the guidebook doesn't say when it was discovered.
Also, in Life and Death Underground, Lovelock ascribes 11 deaths to Alderley Edge (whih would be before the latest one in Wikipedia), & would take it ahead of Porth yr Ogof...