Thanks for the very discouraging but realistic inputs, folks.
It does not seem sensible to take a chance on it when there is a suitable AL flight 1 hr earlier. I shall have to put my thinking cap on pronto and discuss with Air NZ. I fear, however, that the attitude will be "pay up if you want a change" and that is 30 quid a head
AFAIK.
I'll report back if there is a happy outcome.
Regards