That you would Neville! The spanner might be marked 1/4W and 5/16BSF.
For a given size whitform fastener, nuts and bolt heads are 'one size up' in W (or 'one down' in BSF or BSC) - it runs according to shank diameters not the AF measurement. BSW needs chunkier nuts 'cos the coarse thread cuts deep.
There do seem to be cheapo spanners out there with just the one marking these days though.