If you don't have a night qualification
on your PPL, you can cover the material as part of your CPL course, albeit almost certainly at rather greater expense than if you'd done it earlier.
The course consists of a minimum of 25 hours of flying training (30 hours for
applicants without a night flying qualification (aeroplane)) and 250 hours of
theoretical knowledge instruction. The flying training may be reduced by 10 hours
for holders of valid Instrument Ratings or of a Course Completion Certificate for the
Basic Instrument Flight Module.
(CAP 804, p91)