This may be completely irrelevant, but should provide some amusement to those with more experience out there.
After I changed my gearbox this summer, I had the same starting problem. It had been on the stands for a few months with a flat battery which was already pretty old. So I replaced the battery - still wouldn't turn. Replaced the starter motor (for which you need the arms of a nimble octopus) - still just clicked. I then did what I should have done in the first place and checked the electrics. It turns out that I had forgotten to bolt the earth strap back on the gearbox. It was just resting against it, so there was enough juice to put light up the electrics, but starter motor couldn't pull any real amps. Oh how we laughed....
Only relevant if you are a numpty seventh dan like me.