glosgirl
Tricky one that. You clearly have reasons for fixing the date of your visit to a St Athan airshow.
The trouble is that the first public demonstrations of the sonic boom were made at the Farnborough SBAC show in September 1952. (Hunter, Swift and DH110)
The first sonic boom in the UK was made by Flt Lt Taffy Eccleston an RAE test pilot on the Aerodynamics Research Flight at Farnborough some months earlier (perhaps even late 1951) when flying a US F86 Sabre that was on loan to RAE. Taffy sadly died in the prototype Victor a couple of years later but I have just spoken (for a second opinion) to one of his fellow test pilots at Farnborough who knew Taffy well and he confirmed what I have reported here.
So history is puzzled as to how a sonic boom could have been heard at St Athan in public prior to June 1950
Regards
JF