In reply to DeanB:
The current entry on the BMC RAD for Upper Pen Trwyn reads as follows: "Due to the increased number of visitors who are likley [sic] to be exposed to falling rock, climbing during bank holiday weekends and during the period July 15th to Septemebr [sic] 8th is restricted until after 6pm in the evenings. The Mumbo-Jumbo and Parisellas Cave areas are not affected by these restrictions."
The access notes in the excellent North Wales Limestone Definitive Guide by Pete Harrison and Andy Boorman (p. 221) reads as follows: "From Mayfair Wall to The End of the Road, no climbing is allowed before 6 pm on all Bank Holiday weekends and during the School Summer Holiday period (July 15th to September 8th). This does not affect the Mumbo Jumbo or Dumbell Flyer Walls."
Both of these entries therefore imply that climbing is allowed on Upper Pen Trwyn prior to 6 pm during school holidays that are not the main summer holiday and that do not coincide with bank holidays. Can anybody in-the-know confirm that this implication is indeed correct?