Harold Rich at the piano and John McCulloch on bass.
Colin Pierpoint added the following information on the Pebble Mill Facebook page:
Harold Rich was a great broadcaster, and a nice guy. I remember him in Studio 6 at Carpenter Road. He knew the Type A studio equipment so well that he was passing useful information in private back to the control cubicle, by putting on cans and speaking quietly into the piano mic. He knew that the SM (Dave Welsby) could reply on Transmission Talkback and nobody else would hear the conversation. That way he was talking about the vocalist, and suggesting how to improve her performance.