Update:So, with the tools kindly loaned to me by Greg, I popped the cranks off (which showed just how much the previous axle was)
And swapped the BBs over. I'm still not sure on this 73mm vs 68mm thing. The 456 needs a 68mm bottom bracked according to the specifications. I was assuming this was diameter, so there was no chance a 73mm one one even fit in the hole. But somehow, it did. Not all the way, it's a bit of a bodge, but it certainly works! In the future I'll upgrade it to a hollowtech affair, but for now it's fine!
A bit of tweaking later and the front mech was shifting very nicely across all 3 rings, rear mech seemed fine too, and I was ready to go on the inaugural ride. Covered about 2 miles, got to the first climb, then all hell broke lose! Shifted to the largest rear cog, at which point there was a catastrophic crunching, I came to a dead stop faster than I could unclip, and went over sideways. This is what happened:
It doesn't look too bad, but it's completely stuck around a sticky-outy bit on the casette. On the plus side, it was a very nice place to break down:
On the other hand, I then had to carry my not-so-light new ride all the way home, and the rear wheel can't turn at all
The saga continues. This week's lesson was that you should learn how to set up a rear mech properly,
BEFORE TRYING TO RIDE IT.