Adrian,

Try the DOLLARDE function (2007/2010). It converts a fraction into decimals. For example, each of the following functions would return a value of 1.5 (assumes fraction is in cell A1):

=DOLLARDE(6/4, 1)
=DOLLARDE(0 6/4, 1)
=DOLLARDE(A1, 1)

If you use cell references, be careful to ensure that the cells containing fractions are not perceived by Excel to be dates. You can accomplish this by formatting the cells as Fractions or ensuring that the fractions are entered with a "number, space, fraction" syntax such as:

0 6/4

Hope this helps,

Ben