The link above is to the 2 day course not the Certificate of Competence Exam.
Can I suggest you buy yourself a RYA Powerboat Logbook, in it you will find the syllabus for both the course and the exam along with heaps of other info you will need if you are going to become an Instructor
There is no requirement to have done the 2 day course to take the exam but it is quite unusual. The exam is tougher than the course content. The exam essentially covers three areas
- Boat Handling
- General knowledge (IRPCS, navigation, safety etc)
- Night Pilotage
Based on what you told me by pm a month or 2 ago I would have thought night pilotage and navigation would be the areas for you to work on.