Herbert and Gladys do not seem to have stayed in NZ very long - they left London for Wellington on 22 January 1954 and arrived back in London on 8 September 1954. Home address 187 Buxton Rd, Stockport, Cheshire. Their intended future residence was England in both documents, so it looks as though it was just a holiday.
The other Howard on the outbound passenger list - Grace - was from Ilford in Essex, future permanent residence NZ. She travelled on a separate ticket and there is no indication that she was connected to Herbert and Gladys. I don’t see the Dorothy Forster Howard referred to?