Which North?
You will find references to three different norths in the margin of OS maps. True north is just that – a direct line to the North Pole and the Earth’s spin axis. You will be very interested in this if aligning an astronomical telecope. Grid north is as defined by the grid lines on the map which cannot match true north because the map is a flat representation of a curved surface. The north-south grid lines point west of true north for areas west of the true origin at 2 degrees west, and east for those to the east but this difference is only about 0.75 degrees per degree of longitude.
Magnetic North is the direction indicated by a magnetic compass; it does not coincide with true or grid north and wanders from year to year. Its variation differs across the country but with localised anomalies due to geological factors so any calculations can only be approximate. Furthermore, magnetic storms, often coinciding with the 11 year solar sunspot cycle, can produce large short lived disturbances (eg an 8˚ change in less than an hour was recorded in 1989 at Lerwick). Variation in the UK is small compared with that in many parts of the world but not quite small enough to be ignored – it was 4 degrees west of grid in Wales in 1996, decreasing by a degree every 8 years. I hope to be still walking when it’s zero! Whitaker’s Almanack documents this variation in Britain, is published annually and should be found in the reference section of any public library. Alternatively, the World Magnetic Model Calculator hosted by the British Geological Survey site is a useful resource.
This difference between magnetic and grid bearings is called magnetic variation in Britain and in nautical circles but declination in US. Neither term is actually printed in the margins of the OS maps. To add to the confusion, declination is used to describe the latitude of stars (right ascension for longitude). In Britain, it also describes the vertical angle between the horizontal and the earth’s magnetic field (magnetic dip in the US?). We are divided by a common language!