Radon in caves was a news item in the caving press in 1988-89 with Peak Cavern repeatedly mentioned as it was one of the monitoring sites of John Gunn et. al.
Descent 84, Oct/Nov 1988, pp.5 and 28 reported John Gunn and Dave Prime's lecture on radon in caves at that year's BCRA Conference and mentioned that the local press had already picked up the new and run a story titled "Killer radon gas in Peak caves". In the next issue, no.85, Dec 1988 / Jan 1989, p.18 Chris Howes offered extended commentary on the issue. Then in no.86, Feb/Mar 1989, pp.26-7, Gunn & Prime with Stan Fletcher provided a lengthy article which in common with all the previous ones specifically mentioned a few key Peak sites including Peak Cavern and suggested that as cave guides were employees, then radon exposure was legally important to show cave operators in ways that were not relevant to leisure cavers.
I guess that this surge of attention to radon and Peak Cavern being one of the key monitoring sites may have influenced the show cave operators to modify the tour route around this time to ensure that employees did not exceed certain dosages.