If I have this right, the difference between grid and magnetic north is appr. 3 west on the west coast, and 2 west on the east coast?

There is a useful little site for magnetic declination - the UK points of reference are HERE Clicking on any place name calls up a map. Click at any point on the map, and it will give you the magnetic declination for that point. :relaxed:

What a great resource! And it works for the whole world, not just Great Britain =)

Here's another one. OS maps get the data from these guys I believe.

http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html