#jonsmith.... You're correct, it's a latent defect and should be repaired irrespective of warrantee.
The exact same thing happened to my RH750, mute went first then hairline cracks found in all. Took it into Real Electronics yesterday as I was passing Sheffield, they replaced them all in about half an hour. They believe there was a "moulding issue" with the originals and so far as they know, the replacements are fine and they've not seen any returned for further replacement.
Whilst there they replaced all of the rotary knobs too even though none appeared faulty to me, they said it was just a matter of time before the rubber fins split and the knobs became loose.
I'd be more than happy to use real electronics again, great service in my opinion.
Rob.