The bike holder that I bought for £40 arrived today. I assembled it (there's no instructions) apart from the three pieces that just don't seem to go anywhere or have any purpose.
Then I tried to use it.
No matter which way round I set the gripper, it couldn't hold the bike.
Eventually, I gave up and turned the bike upside down and worked on it in the usual way - I replaced both inner tubes with "thornproof" and put in the Weldtite gel inserts; that combination has meant that I didn't get a puncture over the last 12 months.
Now all I have to do is think of a use for the bike holder.