Hello Sue
I have cruised several times, and visited NZ, but none of those was with Just You - I have only visited European destinations with JY. I love cruising, and life on board, but I think this one will be different from those I've done before - this is more an expedition than a cruise!
If you go to the Hurtigruten website, you can download the MS Raold Amundsen handbook, which gives all sorts of info and tips. You're not expected to dress up for dinner - things are more informal and casual wear seems fine. Other ships I've travelled on have had comfortable well-controlled temperatures inside, so that shouldn't be a problem on this new ship, although of course we'll have to wrap up to go out on deck!
All the clothing advice is 'take layers' - for outdoor wear I have base layer sets (thin wool tops and long-johns), a couple of fleeces, waterproof gloves with liners, warm hat and neck funnel, waterproof trousers that go over other trousers. The fleeces and wool tops are ok on their own for indoor wear. And a warm down jacket of course! I think the jacket that Hurtigruten provides is wind/waterproof but not that warm.
For evenings I have slightly smarter trousers and a couple of tops, all lightweight material and casual. I'm taking one skirt for Christmas Day. Hopefully these versatile layerable clothes will make my suitcase a bit lighter than it usually is when I go on holiday!
Do have a look at the Hurtigruten handbook, I found it really helpful and interesting.
Looking forward to meeting you and Una at Heathrow.
Bee