Thanks Adrian and Bill. It does indeed say that up to 24 hours out of the fridge is OK...I had read that previously as one hour. That could have made a difference to our recent short break for our wedding anniversary! Duh. Also useful to read Adrian's comment in the other linked thread that you have used Invicorp "days after being out of the fridge and it's still worked for me".
I had considered using a flask in the fridge. Maybe the flask could go in the freezer to stay cold a bit longer, but I would wrap the vials in tissue or something to protect them from freezing.
Thanks for the link in the other conversation that tells me I can take hypodermic syringes in hand luggage. I had assumed they would be banned and have to go in the hold. So that is useful.