Kudos to Orcon in the end (especially the reps here).
Although when I did a follow up call today to see if a decision had been made nobody new what I was talking about and had to explain the situation again, to two different people, before being transferred to NZ, but they have accepted my request to cancel under the fibre guarantee.
I really look forward to never having to dial that 0800 number ever again!
This thread can now be closed.