Well I appear to have fixed it 

It turns out that all it needed was the reset button to be pressed, but the pen nib I was using wasn't the right size so I used a pin instead and hey presto 

Oh, and the reason the light didn't come on when I connected it to my computer was because it also needs to be plugged into the mains at the same time which I didn't realise 

Thanks for the help anyway.  Glad it works now as I have more important things to be spending money on at the moment instead of a new sat nav!  It probably didn't like being left in the car at the airport with temperatures of minus whatever they were. 
