Silas, Admiralty charts show lines of magnetic variation on them. Of course these are likely to cover only coastal areas!
More seriously, I would imagine a phone call to an HMSO office would tell you if maps are produced showing magnetic variation. Equally, I'm sure that one of the other list members will come up with an internet site where you put in your co-ordinates and get back the variation ;-) Actually I've just looked up quite a nice site if you don't need to be too precise http://www.webworkshop.com/pangolin/magvar.html This shows a Mecator map of the world and displays the latitude / longitude and magnetic variation of the point at which the mouse cursor is. The accuracy seems to be limited by how well you can position the mouse over your chosen location, given that no cities are shown. However, as the variation looks as though it changes reasonably slowly, with atlas in one hand and mouse in the other, I should think you would be able to get variation to at least a quarter of a degree (I didn't spend that long trying the site out as I'm at work). David Higgon London
