I've done some more research. After the first hub failed the bikeshop suggested sending just the hub internals (i.e. axle and gear mechanism) to save weight on postage. I posted the internals off and waited for the replacement.
When it arrived unfortunately it was the 175mm axle unit (my hub has the integrated drum brake and requires the 185mm axle). The bikeshop sent a replacement 185mm unit when I phoned them and told me to keep the 175mm unit (as spares). I've no complaints about the service from the bikeshop (so far).
I've now dismantled the 175mm unit and had a look at the internals. There is a fundamental difference between my "broken" 185mm unit and the "replacement" 175mm unit.
The gear pawls (used to lock the planetary carrier to the ring to achieve unitary drive when the sun gear is unlocked from the axle) are different: on the broken unit the pawl has two teeth which sit over a tooth in the ring gear. The replacement pawl instead has a single tooth which sits in the valley between two adjacent teeth in the ring gear.
So my question is now: given that the design of the pawl has changed, which is the new design - the pawl with the two teeth (the broken part) or the pawl with a single tooth (the replacement part) ?
Look forward to some feedback.
Last edited by fastbike; 10-13-10 at 02:21 AM.