That was my first thought too, but even with lots of adjusting of both the shift cable and the stop screw it didn't affect the problem. It is odd that it's just the 9th cog, which is why I will be hunting very carefully in case I dropped a spacer somewhere, for between the 8th and 9th cog, but I was very careful...
It doesn't *not* fit, it just doesn't sit into the slot as easily, so tends to ride up. I'll take a look tonight at why it's different on the other sprockets, and also whether the 9th sprocket offset is odd. Mind you, all the original links work perfectly/

I'll take a caliper to the pins and check them against the other pins, but I'm pretty sure they'll be longer, not least because the HG53 pin ends are flush with the plates, unlike say the HG73 which looks (from photos) to have sticky-out-bits, for want of a better term!

Cheers
Jon