Yes. I'm afraid that if you don't complete all of the exams in eighteen months then you will have to retake the ones that are out of date. Eighteen months is quite a long time though. If you really want your licence then it shouldn't take more than eighteen months to complete the exams. Twelve months should be more than enough time.
Once you've completed the exams they are then valid for a further two years. In other words, from the date of your last exam you then have a further two years to pass the Skills Test.
Good luck.