Laguna + Red Sat Nav Signal, how can I diagnose?

We have a '52 Renault Laguna the Sat Nav can longer pick up a signal (red light on Sat dish). If you go into the map it indicates we are in the middle of the North Sea!! I checked the disc drive and CD all ok, the Sat Nav unit itself ok (no PCB marks or overheats). The only thing I have not or do not know how to check is if the GPS antenna is working correctly. I appreciate this may not be the problem or source, so any ideas would be greatful.