You're in the UK for more than 3 weeks usually - its generally about 5 I think. If you fail one, you resit it in the next module when you return from NZ for good if the subject was one of the first eight, or you have to head to a testing centre from Bournemouth once you are based back there if it was in the second module. Not entirey sure of the details and what the procedure is, but you are able to resit (although obviously not an ideal scenario).