To the best of my knowledge, the night qualification, or alternatively, evidence of having done the required night training, is a requirement for CPL issue. If you have a CPL, then as far as the CAA are concerned, you are allowed to fly at night.
If you are happy that you are a safe night-pilot (due to your South African training), and the CAA are happy that you are a legal night-pilot (which they must be to have issued your CPL, although they may have made a mistake), then there is an argument which suggests you should keep quiet about the mistake. Of course, I couldn't condone that.
If you decide to do the honest thing, the rules won't tell you what you need to do, because it's not a situation which you should have been allowed to get yourself into. But I don't think you'd have too much trouble finding a night instructor to check you out and then authorise you to do 5 take-offs and full-stop landings. Since you would be flying under the supervision of the instructor, the legality of your license wouldn't matter for this flight, and once the flight is complete you would have met the requirements and your license would then be correct. But the best thing to do is probably get that in writing from the CAA.
FFF
-------------