OS maps are not WGS 84 but the aeronautical overprint uses the WGS84 datum
Jan,
For clarification, "OS maps" means British topographical maps. These maps use a different geodetic datum than the WGS84 datum used by GPS and aeronautical maps. They have a grid of Eastings and Northings in metres but also have a latitude and longitude grid (marked with small crosses).
The aeronautical overprint is adjusted so that a point specified by a WGS84 lat/long is over the correct ground position on the map, but if you were to find its lat/long on the OS map, the value would be different, because the OS map has a different datum.
Clear as mud?
