Nantonos: That library looks like the best way to go. I haven't had chance to try it out yet and as it happens I'm probably not going to get time for at least 2-5 days. I will update when I have had chance.
wwbrown:
Maybe the easiest thing to do is to get g different GPS that outputs the declination.
Easiest and most accurate, true. I've already soldered up my GPS unit and it wasn't too cheap. If I was to start out again I'd probably go for a unit that can output declination.
mmcp42:
oops
sorry for mis-information
better to have everyone think you're a fool than to open your mouth and prove it
Hardly! I've done far worse things 
.
TeslaIaint:
Why not just find out what the magnetic declination is for your area and adjust for it in your code? For example: my location in Ohio is 8 degrees 15 minutes west of grid or true north so I would just add 8 to my azimuth to find grid north. Depending you exact your direction needs to be, you could use decimals. Don't forget to convert minutes to a decimal. multiply by 1/60This will use your ip address to give your magnetic declination:
http://magnetic-declination.com/
The unit will be mounted on a vehicle without internet access and toured all over Europe. As I said earlier though: "Looking at the magnetic declination currently I see that from Agadir (western Morocco) to Istanbul (eastern Europe) and from Oslo (northern Europe) down to Nairobi (central Africa) the magnetic declination only varies from -8 deg to +4 deg. If we didn't correct for it at all our panel would be 8 degrees out in its positioning which would still provide very nearly cos(8 deg) = 0.990... of the energy falling on it."