Navman GPS

Are there any Navman experts out there? This question relates to a boat not a 4x4 sorry! I am up at Onslow at the moment and have just installed a brand new Navman Trackfish 6500 in my son’s fibreglass boat. All the functions e.g. chart, sonar, etc are operating but we can’t get the GPS function to operate at all. Just what you want at Easter  We have followed the GPS set up instructions as explained in the manual including Restart GPS but the unit refuses to lock on to the satellites and just keeps displaying the “No GPS” message. It doesn’t even seem as if it is going through any search process at all with the “No GPS’ message coming up the instant that the last instruction of the restart GPS procedure is input. It’s as if the GPS aerial cable is not plugged in to the unit at all. Obviously there could be a fault in a brand new unit but in the event we have missed something important in the instructions I just thought there may be someone out there with more experience with these units than I have, this being the first one I have had anything to do with, who could maybe offer some suggestions as to what we might be doing wrong.