You need to do a 55 hour IR course if you have a PPL. However, I don't think you can actually take the test at the end of the course without having first passed all of the theoretical exams first. It may not be worthwhile doing the course under these conditions, as it would mean having to take the test at a later date and you may no longer be up to speed.
It's not that unusual to do the IR before the CPL though, as it does mean that you then only have to do a 15 hour CPL course.
Maybe look up the finer points in LASORS?
http://www.caa.co.uk/docs/175/Lasors_Section_E.pdf