Differential cryptanalysis in "The Block Cipher Companion"
I haven't studied this in detail, but the authors seem to give quite a comprehensive presentation of the differential attack.

Please note that this cipher is essentially the same as the one in Heys' Tutorial on Linear and Differential Cryptanalysis (easily found online by a google search) but the Sbox is different.

I have had students implement that attack successfully. So you can use that text as an extra reference while you are trying to sort things out.

In fact the authors themselves of the Block Cipher Companion state, on page 121, that 


  The probability of
  the characteristic we have identified could be too small to effectively distinguish the
  correct key information.