Mighty goose
I TOTALLY agree with gazb2 if you do not feel well prepared! Remember, NATS are looking for fully prepared candidates as opposed to just making the numbers so the supposed 'opportunity' would be of no benefit to you at all if you were to underperform due to lack of adequate time to prepare.
Even NATS I believe would advise you of the same should you need to re-schedule!
You are so close now, don't throw it away except ofcourse you are confident and feel adequately prepared.