Today I fitted the retro-fit armrest that I asked the mini dealer to supply when I picked the car up over the weekend - I hadn't even got it out of the box when I posted yesterday.
Dead easy, just remove a torx bolt on each seat runner, and slide the armrest bracket under. Reassembly is the reverse of removal

I changed the front seats today as well, so while they're out it's even easier.
It's not the factory-fit armrest, the bracket is just black painted steel, but you can't see the bracket closely unless you tilt the front seat forward. Came in a mini box anyway.
30 mins absolute max including tea and finding the right torx bit in the depths of my garage