Thanks both for the advice. I'm using an adapter similar to the one in your pic Captain_zzap (small black powered dongle).
I have tried the various modes suggested to no avail. Interestingly, I notice that whilst I do get a picture of the main menu on the ACA500+ and also the profile screen (F10), every 10-15 seconds, the screen does go blank for a coupe of seconds. Clearly this dongle doesnt work correctly. I'll have to research a 'better unit'.
Thanks again for the help.