I found a solution - unplug from mains, remove button cell (backup) battery from unit and leave it 20 mins.

When you plug it back in the unit will have reset and will scan for stations and obtain the time/date. Reinsert the backup batttery and the unit should work again (mine did).

Not sure what caused this freeze - suspect the DAB radio got stuck on a station with a weak signal and this caused some sort of software error.