Apparently the spray painting is all about having enough CFM from the compressor rather than the tank volume.
I just ordered a 9.6CFM with spray gun from SGS Engineering - or rather thought I did until a different compressor turned up with a nail gun. About to shout angry things at SGS when I double checked my order. I must have clicked on the wrong picture without realising it. SGS were very nice and agreed to swap for the correct one if I send it back to them.