For JAR Licence training/application you cannot test your own student without specific written permission from the CAA - the CAA will normally only allow this permission if you have flown with the student on one or two occassions maximum for the purpose of a "currency check" or "progress check" not for specific flight training. Occassionally the CAA allows permission for examiners to test their own students for the grant of a JAR PPL if the school/student is located in an area that is very low on examiners and where another examiner is not readily available.
For National Licences the Examiner can test their own students, however for both NPPL and IMC it is worded in Lasors and/or the FE handbook (I don't have the FE handbook to hand for a reference or exact quote) that the examiner should consider how much flying they have done with that student and whether it is suitable to test the applicant.
Both of these bits of legislation is for training for the gain of a licence, and therefore an examiner can test a previous student for the purpose of a SEP renewal/reval or for a subsequent rating e.g. an IMC rating as long as the above restrictions are met etc (however once again the examiner should take into account how much flying they have done with the applicant/candidate)
All the information you need is in LASORS, the Flight Examiner Handbook and the relevant CAA Standards Docs for the licence or rating course you are instructing/examining. If you are still unsure speak to another examiner higher up the pecking order than yourself or phone the CAA, however phoning the CAA directly will normally just mean you will just be reference backed to LASORS or the FE handbook.
Best Wishes
Scott