but weirdly I still refer to 1/4 whit when I need a 5/16 BSF, so I bought a confused set of spanners marked to suit.
There's a story there. Pre-WW1, 1/4BSW bolts and nuts all had the hex size of a 1/4W spanner. Then someone realised that there was really no need for such a big hex, saving a lot of metal, so most post-WW1 British fasteners have a hex one size smaller. All 5/16" fasteners for the last 100-odd years (5/16W, 5/16BSF, 5/16BSCy) have a 1/4BSW hexagons.
Because the BSF and BSCy Standards were written long after the BSW Standard, they incorporated a smaller hex in the Standard. Thus the spanner labelled "1/4W / 5/16 BSF".
So if you buy a 1/4W bolt and nut these days it is NOT 1/4BSW. Some wankers like me collect REAL Whitworth nuts - large hex - to go on things like early Sturmey Archer gearbox studs (3/8 BSW), or all the small fasteners on my 1909 Lewis (1/4 BSW).
Leon