Using OS data and CRS of EPSG : 27700 on QGIS, on the new Print Composer how do I get the correct Longitude and latitude scale on the outside of the map? at the moment both scales on the X:Y axis start at 0, not as the actual map states?
You need to change the CRS of your grid (see image below). My map is in OSGB National Grid (epsg:27700) but the grid can be in any projection I like, in this case I've set it to EPSG:4326 which results in the pleasantly curved longitude and latitude lines crossing my map.
For best results I also find it useful to modify the draw coordinates setting to be just latitude on the sides and just longitude on the top and bottom. This avoids confusion where the longitude lines cross the sides of the map.