Hi there. It was the 5th gear synchro; a known issue on the Zed. There have been quite a few warranty replacements at under 60k. Expected life expectancy of a clutch is 60-100,000 miles. It makes sense to replace the flywheel if replacing the gearbox (as I paid no more in labour to have this done). I could have got it fixed (probably for less, but would have taken longer). Car basically has a new drivetrain now; virtually all other consumable parts (except engine) have been replaced. The expected lifetime of the VQ35de is 200,000 miles.
The gearbox was £460 for CD09 (latest edition, with revised synchros). Fitting was £275 + VAT. The rest was just parts other than the gearbox, which I could have got a better deal on if it wasn't Christmas, and I would have had more suppliers to choose from; I had to pay £500+VAT for the flywheel, which isn't a good deal.
Most of the costs on my car have been at the 60,000 mark.
In summary, I think that my car is in better condition now than most at the 55,000 mark. The cost of repair was less than most cars depreciate in a few months nowadays, and I think I am now on the flat of the depreciation curve (new/newer cars depreciate faster).