If you experience poor DAB reception while you have an iPod plugged in to your Highway 300Di (for example to charge it), you should ensure you are using an official Apple iPod accessory cable for your connection. Unbranded (non-Apple) accessory cables that are not sufficiently shielded may cause radiated RF interference which will degrade your DAB signal.
You can purchase official Apple accessories from http://store.apple.com/uk