To summarise in case anyone has the same issue..
I didn't realise that two devices with the same map version number could have different detail levels. I assumed when it said there was a partial map of Europe installed that it referred to the area and not the detail in the map... My mistake. So it was doing it's best to route along the via points but as it was missing a load of the roads it was never going to end well.
The two devices comment was me trying to send routes through from Basecamp. I think it had corrupted them when they were uploaded to the 660 and couldn't then route correctly.... only a theory..
Anyway thanks for all the help. Much appreciated.
If you have a map loaded on your device it should have full detail, a problem might occur if a device only has a basemap for one area but otherwise "detail level" is only relevant to the maps on Mapsource or Basecamp.
John