I got the old armrest off. The plastic bits have a tiny allen headed grub screw as part of the retaining mechanism. The chap in that video said it was a 3/32". I tried all my allen keys and the only thing that would fit was the long ball end of a 2.5mm VAR key, part of a set I bought from "Hard To Find" years ago. I had to use pliers on the angled bit to get enough leverage, they were pretty tight.
New bits now ordered