if you live in the manchester area theres a place near Hyde called "Brands at home". Or Dunhelm (think thats how its spelt) certainly have branches in the northwest, dont know about nationwide though, try google, I know they have a website. Sure that if you went to a bedding place they could order one for you. lol
Bigger than a king is super king and that is basically two singles pushed together size. Yes I thought queen was smaller between double and king ..one would asume.
I just got a super king mattress protector from a shop called Derrys near me but they do sell two types in argos in super king. They also sell super king duvets but do not sell duvet covers to fit...pity.
There`s a company called Keys of Clacton who specialise in all bedding and in unusual sizes/ shapes etc. If you google the name you will get their web page and you can order from there or by ordinary mail order.