Turns out the cage was broken and one of the balls had popped out and was trapped in the boot. I caught it before it failed spectacularly. Because there was a cable tie on the boot and I couldn't find the broken bit of cage I suspect it was already broken and has been semi ok for all of my ownership.The problem is I can't seem to find a replacement one anywhere in UK.
There are some rather expensive uprated options from the states but a lot of those places don't want to ship here any more because of brexit. ATS says they will ship but UPS has been losing a lot of packages.Even a replacement cage would be ok as my inner and outer race are good.