If the lockring has come off as theorized, all you'll need to do is take the wheel off and all the cogs should come freely off the hub.
I took the wheel off, and this did indeed happen. The entire thing fell off. I had cogs hanging off the bolt holding the wheel in place.
Hmm - interesting, but if it's something like busted pawls in the FH, wouldn't it show the same symptoms using *all* the sprockets, and not just the smallest 1 or 2?
I don't think I explained myself that well. I only tried changing the chain onto those two cogs. When both of them spun round, I thought I was damaging the cassette more so stopped. If I did move the chain onto any of the other cogs, they would have spun round as well.
Interestingly, my bottom cog is missing a tooth. I don't know where that went, if it was today or earlier, but i've only had this thing about a month. I haven't pedalled particularly violently or anything.
Also, I run my bike in single speed on a cog with 14 teeth (this is the cog that is missing a tooth). Would I be able to swap my entire 6 speed cassette for
this? I'm unsure since they also list a
14 tooth cog but say it's compatible with 7 speed, but i've been running my bike on a 14 tooth cog on a 6 speed.
Finally, is
this the tool I need?
It seems like there's going to be a DIY say in my future.
EDIT: Also, when I installed the cassette, I just screwed it on with my hands. I didn't tighten it with anything. Was this right? I did notice that when I put the bike upside down and spun the back wheel, the cassette did wobble slightly. My back wheel also wobbles slightly too, even when screwed in tightly. Are both these things meant to happen?