Thanks for the feedback everyone.

I tried Todd's suggestion of minor changes to the guilty methods, but
didn't see any change to the status of the classes - I suspect that
the references may be too obscured for me to uncover them all.

Running Bert's fix removed the immediate issue, but then RB had a
problem with anObsoleteT1 (Trait) created by the same suite of tests.
This appears to be totally unreferenced and I don't understand Traits
at all so I'm ignoring it, as I've come up with a fix for RB.

So as Jerome suggested I've raised the bug on Mantis (#5910).

I've added a simple fix to RB as bug #5911 which makes it exclude
obsolete classes from its consideration. This allows RB to refactor
successfully, but I can't guarantee that it doesn't introduce any
issues.

Cheers,
Michael