Further to Mik Moore's reply:
You are quite right: you can have 4 attempts at any single paper within a total of 6 sittings to pass all 14 papers. The 14 papers must be completed within 18 months of sitting your first paper.
However, if you fail a paper 4 times, or you run out of sittings, you are allowed to start all over again, but you will be required to take further training from a JAR Approved School. Once again, you would have 18 months to complete the 14 papers successfully.
As I understand it, you could do that as many times as it takes, tho' I feel most people would be discouraged from continuing if this happened to them.
Hope this helps.......
Martin
(ex-PPSC Gen Nav Instructor)