Some additional feedback on the Sennheisers after my first commute:
1. Pairing: I took them out of the case this morning and they didn't reconnect. I couldn't pair them - in the end I had to 'forget' them from my phone and repair. Quite annoying. Thankfully it worked on my 3rd attempt, but irritating all the same.
2. I used the app for the first time for real, and found that despite the fact that I was listening to music quite happily, it couldn't connect to the earphones. What? I guess the headphones weren't responding or something over a separate BT channel. Really weird and annoying.
3. Battery life: After around 2 hours, the left was at 66% and the right at 38%. That's on-target to hit approx 4 hours (based on the master's max battery life), but it's a bit unnerving to see such a huge amount of difference. Anything more than 3 hours is plenty of me though, so whatever. Also, popping them into the case for around 15-20 mins increased the charge from 38% to 66%, so they charge pretty quickly.
4. Transparent mode works well, but I'd really like to configure the double-tap on the right earpiece to be a single-tap instead (since I don't use voice assistant).
5. Sound: All of the above niggles are completely negligible because of the fantastic SQ. Watched 'Venom' on the train, and it sounded sooooo good.
Overall, very pleased. However, I hope these niggles get ironed out in an app/firmware update in future.