Panic over, have worked it out. The car has two choices for connectivity for GraceNote lookup. It can either use its internal DCM unit or if available, WiFi. For the lookup to actually work you need to have a strong enough DAB signal to get the RDS data back, this is then used over the DCM unit (or WiFi) to lookup and download album art. If you don't have both the strong RDS signal and also a connection over either DCM or WiFi, album art lookup will fail and it will just show the Radio Station logo.