or.........when you fit the new bushes, before fitting the assembly to the bike put 3 or 4 dot punches around the circumference of each bush/carrier hole. I've also used bearing retainer in the past which seems to do the job to. Makes them a real pig to get out then.
If you can't get the stripped one out. Remove it with the carrier, get it in a vice and undo it that way.

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I'm now confused!!!

The circlip groove looks OK - the circlip can be rotated, but is not loose and moving in and out at all.

The cush drive bushes/silent running blocks are retained by nuts on the outside of the sproket carrier, so why would a worn circlip groove make the collars detatch from the rubbers and move out of the back of the sprocket?

That's the problem - while it looks ok, chances are the it is worn out of spec enough to start backing out. This is what happened to me. Everything looked fine and went back together fine when replacing the chain & sprockets. I even had a quick release carrier with the lip in it to stop them backing out but it still went!

Just keep an eye on it as it might happen again...