STurns - if you hold more than 600 hours, have passed the CAA written exams and hold a CAA licence, you are exempt from the approved course - you simply need to reach the standard required to pass the test. This was/is the route commonly taken by flying instructors building hours before getting an airline job. On average it probably cost you around half of an approved course.
This situation is not applicable to you so you must do an approved course. ickle black box was asking about adding an IR to his JAR-FCL PPL, which means a 55 hour approved IR course. I was only really talking a ballpark figure, but I can't think of any places where you could get one much cheaper than £10,000.