Any question answered...

Clerkenwell Screw is around the corner from my office, but I hoped not to have to go down and try every bolt of about the right size. Although they are very helpful.

Every bolt? only need to try out a couple and see if it fit.

re the BB, I assume you mean to move the front chainrings into a better line with the rear cassette - middle to middle as it were.

Exactly, by putting a shorter bottom bracket on, have a look at the chainring/crank arm clearance on your bike to see if you can bring it 2 or 3mm closer than usual if you can't get the derailleur to stop rubbing at the highest/lowest cog.

Don't necessary need to be middle to middle thought.