CIS - am I missing something

Please excuse the back of a fag packet calculation but it should suffice.

I have a client in the CIS who has earned 40k and had 8k deducted at source. He has roughly 5k expenses for the year

Relevent profit therefore 35k less PA 6.5k = 28.5k. Tax @ 20% = 5.7k

Class 4 - 30k - 5.7k x 8% = 2k

Therefore total liability roughly 8k and 8k deducted at source therefore nothing due

The client is convinced he is due something back (as everyone he works with is...) but I cannot see how. Have I missed something fundamental or are his colleagues maybe being over zealous with their expenses? Any ideas would be happily received

Cheers

Nigel