As experienced navigators will know, magnetic north shown by a compass varies from grid north on a map, and for centuries in Britain magnetic north always lay to to the west of grid north, but since 2014, starting in the south west, magnetic north has started to move to the east, and this change will slowly spread across the whole of Britain over the next 12 to 13 years.
The difference between magnetic north and grid north is often referred to as grid magnetic angle and it not only varies from place to place, but changes with time too, and needs to be taken into account when navigating with a map and compass.
In 2014 there was a significant event in the changing direction of magnetic north relative to grid north on OS maps. For the first time in Great Britain since the 1660s, magnetic north moved from being to the west of grid north to the east. The change started in the very south west corner of Britain, currently affects the areas to the west of the line on our map, and will slowly progress across the whole country over the next 12 to 13 years.
More information can be found here>> Ordnance Survey