Cool, thanks.
I tried out the Nikwax again last night and popped it in the tumble drier to help dry it out, thinking that would also activate the product. But reading that page as a result of your post it seems that it needs tumble drying again, after it has dried out completely. In all honesty it was still a tiny bit wet after I took it out last night.
It's currently hanging up in a room with a dehumidifier so should be bone-dry by the time I finish work. I'll pop it in the dryer again and see how I get on!
Edit: I've asked Mammut UK via Facebook exactly what product they would recommend using on their membrane jackets and they've said "We have a large section on our main website that offers care and cleaning instructions for all clothing and kit. However the product we would recommend the most would be either NikWax or Grangers. Both of these will do the job of cleaning your jacket and then reproofing after"
Another Edit: Sadly, another blast in the tumblr dryer has made no difference. The water almost instantly soaks into the jacket.