Read LASORS E 3.4. If you hold a night rating issued pre-JAR i.e. before 1999 then that will have included 4 hours of IF as a prerequisite to completing the night rating. 3 of those 4 hours were creditable towards the IMC rating. You must therefore include those 3 hours as dual IF training on the IMC application making the total time up to at least 15 hours.
If you have a night qualification issued post JAR i.e. after 1999 then you did not require the additional dual IF therefore there is nothing to credit. If you did any additional dual IF training associated with the night rating then you can count that towards the IMC total. If your application shows less than 15 hours then it will be rejected.
When you completed the application the examiner is required to sign it and he should check that you are eligible.