I hadn't realised that Elecraft had a diversity receive system.
I've just searched via Google and listened to the audio clips on N1EU's Elecraft site. These give an identical performance to what I heard from the new receiver.
I recall thinking while I was testing it to try and make some recordings of the stereo diversity audio. I just ran out of time.
It's got me thinking that adding a second receiver in the Sentinel 5 transceiver would be very easy. Adding the extra soundcard channel into the SDR software wouldn't be so easy but I think it would be worth the effort.