It looks a bit messy but I got it working 
CASE WHEN DD.UNIT_OF_MEASURE_NAME = 'Feet & Inches' THEN CONVERT(DECIMAL(10,2),PD.DISCRETE_DATA_VALUE)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Inches' THEN CAST(FLOOR(PD.DISCRETE_DATA_VALUE / 12) AS VARCHAR)+'.' + CAST(FLOOR(CAST(ROUND(PD.DISCRETE_DATA_VALUE %12,0)AS VARCHAR))AS VARCHAR)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Centimeters' THEN CAST(CAST(PD.DISCRETE_DATA_VALUE / 2.54 AS DECIMAL(10,2))/12 AS VARCHAR(10))
WHEN DD.UNIT_OF_MEASURE_NAME = 'Inches' THEN CAST(CONVERT(DECIMAL(10,2),PD.DISCRETE_DATA_VALUE)AS VARCHAR)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Pounds' THEN CAST(CONVERT(DECIMAL(10,2),PD.DISCRETE_DATA_VALUE)AS VARCHAR)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Ounces' THEN CAST(CAST(PD.DISCRETE_DATA_VALUE/16 AS DECIMAL(10,2))AS VARCHAR)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Kilograms' THEN CAST(CAST(PD.DISCRETE_DATA_VALUE * 2.2 AS DECIMAL(10,2))AS VARCHAR)
WHEN DD.UNIT_OF_MEASURE_NAME = 'Pounds & Ounces' THEN CAST(CONVERT(DECIMAL(10,2),PD.DISCRETE_DATA_VALUE)AS VARCHAR)
END AS FINAL
Thanks for the advice everyone!
Cheers!
M