Er, no, not immature spuds!

A little searching has shown that actually, they're known as aerial tubers. They seem to be symptoms of Rhizoctonia (see http://www.scribd.com/doc/22035248/Rhizoctonia-Canker-and-Black-Scurf-of-Potatoes). The cankers appear on the foliage and the black scurf appears on the tubers. We saw some of this dark sooty deposit on some of our spuds last year but it wasn't apparent on any of these Lady ChristL ones.
There certainly didn't appear to be any adverse affect to the taste, look or quantity of tubers - maybe that's because we pulled them quite early.
Ho hum, just another disease to be aware of this year!!