I think I've finally worked out why some channel names are shown properly on the front panel display whereas some show as D001, D002, etc. It appears to be due to the length of the channel name as it is shown on the blue GuidePlus banner that appears when you change channel. As long as the channel name is less than five characters long (including spaces) it will be shown properly, i.e 'UKHIS' & 'SKY 3' will display properly, but 'BB1ENGL' will be displayed as D001.
Bit pathetic really - someone (Pioneer or GuidePlus) needs to think about this sort of thing a bit more.