After nearly a year of waiting I have now had my two Azur 640T tuners
replaced with new ones with version 3.0 of the firmware. Cambridge
Audio announced on their website that the upgrade was available; see
"DAB problems" in the discussion forums on the CA website.

This upgrade from V2.0 to V3.0 has (it seems) completely cured the
problem where the unit used to lose DAB signal and 'lock out'.

I have also noticed that V3.0 has improved the DAB reception generally.
I now do not need an external aerial; I can get a good signal with a
bit of wire trailed on the floor.

Cambridge Audio do not seem to be recalling the older units, but will
replace them free though the dealer the unit was sold by if the user
requests it. You can check which release you have by pressing and
holding in the 'info' button for a few seconds. This brings up more
information than is displayed in the standard sequence of info items,
such as the firmware version number and the the station bit rate and
'stereo/mono/stereoJ'.

Robert