In case I need it again, the instructions are here, so I thought I would share it in case it helps someone else.
I claim no credit other than being able to Google and then to create a list of instructions, so thank you to the people who made this possible.
Bought a new chair today, a Mirra 2 from Herman Miller, hopefully as comfortable and supportive as the one I sat on in John Lewis.
We found it on WellWorking.co.uk but it was £667. However we saw the original model in John Lewis for over £800, so we are sure we have saved over £200.