St David’s Hall Saturday 17 September 7.30pm   ST DAVID’S HALL is winding the clocks back to the Swinging Sixties as the award-winning production Let’s Hang On comes to town on Saturday 17 September!   Plus, this spectacular show takes you on the unforgettable musical journey of one of the most successful bands of all time – Frankie Valli & The Four Seasons.   The 60s wasn’t just about The Beatles, the Rolling Stones and the Beach Boys as Frankie…