Here's how I overcame / fixed it!
The get you going bodge: Take off the column cowlings. Remove the barrell from the column by undoing the two shear bolts. Mine yielded to my mini molegrips, and a lot of small turns. This will remove the steering lock. Then take the switch off the end of the ignition barrel. As long as you keep the transponder key close to the coil that sits around the end of the end of the barrel where the key goes, you can now operate the starter switch with a screwdriver. Car will run and drive this way.
The 'Fix': I took my locked barrel, held it key end down, and gave it a number of sharp taps around the outside of the barrel. Something brass coloured (the broken bit of the mechanism?) fell out, along with some quite sticky balled up bits of graphite lubricant. Lock barrel now feels better than ever, completely smooth, no catching or sticking at all.
Used a hacksaw to cut slots in the shear bolts so I could do them up with a screwdriver, then re-assemble.
Two days later, and lots of starts, and test operations of the key in the barrel, and everything still works.
Just put the cowlings back on. So it is either done for good or will not play tomorrow morning!