Thanks again all. Airline were bloodly hopeless, they were just like "errrr yeah probably it'll be fine..". They also only have emergency amounts of formula on board so it's probably not worth relying on them...!
Also on the water at 70 degrees, my Public Health Nurse said when she visited Monday that although that is the current guidance and is obviously correct to sterilise the formula powder, it is very very practically difficult to make sure that you are making formula with water at that temperature for every feed, particularly when out and about. She said that due to these practical difficulties, most mums on her books continue to make up formula with cool boiled water in a sterile bottle when out and about (ie tipping in the powder). She said at home though when making up feeds (I make up 6 bottles at once and put in fridge) she said to make sure the water is still very hot. Although how the hell you tell whether it's 70 degrees is hard, my scientist DH says that water would cool to 70 degrees or lower very very quickly. Thanks also for the poster who said that Evian could be used in an emergency, checked this with French friends of mine who said yes indeed that's what they have used (and have been advised to use) from birth with their baby!
Right, so this the plan:
Have put in an order for 20 Aptamil ready to drink cartons with Heathrow Terminal 1 airside Boots. Will have 5-10 in my handluggage coming over from Ireland too, and my backup powder. Just in case anyone else planning on travelling super-longhaul with baby is reading this, Heathrow Shopping Services (who are very helpful by the way) said that because the order was so big they had to confirm with Boots 7 days ahead of time. They didn't say what was the amount of cartons that made it a "big" order.... so I'd say that if you can, always order a week ahead!
Have ordered a Milton Solo Steriliser (and steriliser tabs) which looks fab, fecking annoying though as I had to order from Mothercare UK as not in stock anywhere in Ireland. If it doesn't turn up by Monday (I'm flying next Thursday) will ask airside Boots if they have any in stock/can put one on hold for me. Or will think of something else (hope not). Are those Milton tabs easy enough to break into quarters??? Will see if I can find a little bottle brush and little washing up liquid as per cheungfun suggestion.
Will pack 4 of my Avent bottles. Thinking of buying some of those Chilli Peeps teats for the cartons too, they seem very handy to have anyway! In my checked luggage is going the other bottles, my Avent microwave steriliser (they are about 5x the price in NZ for some stupid reason), microwave steriliser bags (for when not at home with family), and one or two tubs of Aptamil (which is not available in NZ - will figure out when I get there what formula is most similar).
Have got Ikea kitchen containers that I will put backup powder into as per forevergreek suggestion. About to buy 0.5L Thermos for the hot water.
On the suggestion of a friend who's a paediatric nurse will pack Infacol to give before each feed to zap windyness (and therefore crying and associated death stares from other passengers)! Seems to work well on DS!
As for nappies - I am going to take 1 nappy for each hour of travel.... so about 36-40 
. So about half a big bag of Pampers, size 2. Seems like it will fit in my carryon bag fine. Will also take a one pack of wipes, enough nappy sacks, and one of those tiny tubs of Sudocrem. And 6 sleepsuits and 6 vests. A couple of kitchen ziplock bags for putting wet clothes into. And a change of top and pants for me and DMIL. 3-4 muslins. 1 cotton cellular blanket, 1 fleece blanket, 1-2 thin merino blankets.
Will see how enormous my hand luggage is after all of that....! I am taking a standard pull along rolly bag, and my awesome cheap change bag I bought from Amazon.
In terms of transporting baby easily around airports (because the stroller will be checked in from the start) I have borrowed a friend's Baby Bjorn carrier which seems brilliant and comfortable, and DS seems to like. It's so cute, he's only 3.5 weeks old so his legs and arms are very very tiny still hanging out of it! I had bought a Moby wrap previously but to be honest there's so much fabric it seems like a bloody faff dealing with that in an airport - it's about 12 feet of fabric!! (also I can see security telling me to unwrap baby because it covers so much of my body - eg like they tell you to take off big jackets/scarves). Baby Bjorn seems like it will be easier to switch between myself and DMIL who is carrying baby.
Ummmm anything else??? Any other suggestions???