Fly at night - you need a night rating. About 5 hours instruction...
Forgive the pedantry but it's actually now called a night qualification.
Good news is you can do most of (if not all) of it within your minumum number of training hours for the PPL.
That said, however , it is unusual to qualify in UK in the minimum number of hours, best to budget plus 10-20% in costs for your PPL.