Just thought I'd share with you the outcome of this. Took it to Williams today. Whilst the rubber has come away, there is an inner bit to the rubber which goes down the shaft drive by about an inch. This is clipped in. Checked some other bikes whilst I was there and they were the same. Technician told me it would be more obvious if it had completely come away. All good. Nothing to do. White residue is grease as someone said earlier.
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