Seek out some quotes and see which is cheaper as it is doubtful that other Insurers will accept any quaint NU scheme which provides for counting a year as year, before a year has passed.
If your 4 years works out with cheaper quotes for comparable insurance, take out the comparable insurance. £900 + for an annual policy when NU have only been charging £577 for their rapid policy seems a bit steep. That is unless the £577 rapid policy is not for a full 12 months' policy, as you do not explain how the renewal requirement is Sept 08, when your policies appear to have all started in Dec each year?
What do the NU Renewal documents say? If they say 5 years No claims then that is what you must put forward to alternative Insurers, as that is what you have documentary evidence for, from NU - 5 years No Claims, whether it got there from rapid or not.