OK, I'll admit it - sometimes I get things wrong. So a big thanks to both George and Andrew for drawing my attention to the doubtful veracity of that Peter Saville website that I mentioned last week. I have to own up to sometimes just taking things at face value, so I'd assumed that if a typeface is being given away, it must be coming direct from the originator. But if I'd looked more closely I'd have seen, of course, that the font in question is Trajan, which belongs to Adobe and shouldn't be given away at all. (You know, somehow I can't help feeling that when discussion turns to Peter Saville there's always a lot of smoke and mirrors involved.)
But my error does make me go back and dig a little deeper: and the 'J' which ought really have been at the top of my list is Johnston.
It was commissioned by Frank Pick in 1913 and completed in 1916 to become the first alphabet created for a specific corporate identity. Eric Gill, whilst he was a student of Johnston's, also worked on the typeface - and it became a major influence on the later development of Gill Sans. It was redesigned in 1979 by Eiichi Kono at Banks & Miles to produce New Johnston, the variant of the original font currently used by Transport for London. International Typeface Corporation also released a variant in 1999 called ITC Johnston, produced by British type designers Richard Dawson and Dave Farey, which you can read about here.