The is no such thing as a night rating these days under UK JAR. Its a qualification and as such is based on experence training with no final test.
You have completed the required course. It never expires.
It is not uncommon for wannabies to not get it issued then to get the whole lot issued in one go ie CPL/IR/MEP because its cheaper. The night training is a requirement for issue but it can have been done at any point previously or even during the IR