I used the IFixit kit, replaced the battery without removong the motherboard by the way – and it works. It started out saying Zero cycles 100% health. Within an hour is was saying 95% health. I did what I thought was a full recalibration – even though I read this was not needed these days – no change.
The health is now 90%, still well above problem levels, but I'm getting about two hour's life out of it. I expect four ish – was I over estimating the improvement I could expect?
The old battery said 'replace now' from time to time and was getting worse, but still managed about half an hour.
Disapointed… Rightly ?