W/Co Beamont flew the second prototype Sabre PU598 at Muroc in California at Mach 1 plus on 21 May 1948.
This was in straight flight, the DH108 reached Mach 1 in a dive on 9 September 1948 , the DH108 being the third aircraft to reach Mach 1, ( the P-86 and the Bell X1 in the US had beaten it to it).
Further reading,
" The Years Flew Past" by RB. published by Airlife.
" Testing Years" by RB, published by Ian Allan.
Best regards,
om15