Had a go at the Cush Rubbers today as there was a bit of movement starting in the sprocket carrier.

There was still a small bit of movement so I put another piece of inner tube in against the smaller blocks next to the hub.

Now the sprocket carrier is firmed up nicely.
On measuring the depth of the lugs that go between the rubber blocks they are about 25mm & the depth inside the hub itself is about 35mm. So the sprocket carrier lugs are only sitting against a half to two thirds of the Cush rubbers. Maybe that's why they get destroyed so quickly. Hopefully now I won't have to touch them again for a fair while. ( Thanks to Kev ).