OK I picked up a right hand side axle from the wrecker, installed it LHS of my car and put LHS unit from my car onto the RHS. There was nothing wrong with the C-clip, just a totally clapped out CV joint. The job was long but not as hard as I had imagined. The car is running well now - the unit from the wreckers was in much better shape than my old one, and I'm just hoping I don't soon regret not changing both sides.

One tip that worked well for me for removing the C-clip - spin it around and push it off. I figured that it couldn't go far and I'd be able to probe around inside the diff until I found it. I've done it three times now (the first one in the wrecker's yard where it didn't really matter what happened) and each time the clip landed neatly in the diff sump. Too easy!

Another tip: the brass tube that the bolt that holds the axle into the wheel runs through - different models have different lengths! It took me a while to work out why I couldn't put that bolt back through, until I figured that I was using the tube from the wrecker's yard and not the one from my car. There was a good centimetre/half-inch difference in length between the two.

Many thanks for your words of advice.