It's not the CAA who recommend the fee; the main industry price fixers are the FIE panel.
I charge 50% of the JAR-FCL PPL Skill Test Fee for the NPPL NST and the same for the GST. Thus:
JAR-FCL PPL Skill Test: £150
NPPL NST: £75
NPPL GST: £75
That seems fair enough to me as the total time for NST+GST is roughly the same as for JAR-FCL Skill Test.