Thanks once again to everyone for the very useful comments about the DAB mapping.
I have revised the maps and they now show TWO types of coverage area:
The shaded coverage area is -58 dB(uV/m) or better, suitable for "mobile and above first floor DAB listening", the thick line represents -65 dB(uV/m) marking the outer limit of "indoor DAB reception in domestic buildings".
It is probably worth pointing out that these maps are generated from the directional data provided by Ofcom, and do not take into account the effects of terrain as this is not a function that the Google mapping API provides.
Briantist: It looks like it makes sense if you lose those "-" signs you've put in front of the field strength numbers i.e. shaded area is 58dBuV/m and thick line is 65dBuV/m (the thick line being closer to the transmitter and hence having higher power).