Hello Newfy,
Thank you very much again for all your searching.
I think William snr being a gardener may be a red herring, as the same source says his wife is Hannah.
William jnr was definitely a gardener, and collected and described plants for at least one notable NZ botanist, Leonard Cockayne. He worked at several of the government gardens around NZ - we now call them botannical gardens.
Finding the family in the different census also confirmed the childrens' names being handed down.
I will now start seriously searching at my end for his arrival in New Zealand.
Thank you for all the work you do for all Rootschatters.
Jean