Seems like a great price to me! Plus Costco will dispose of your old mattress for free. All sizes are avaiable and all at great prices!
Edit: Seems like NO TIME LIMIT on returns, either!
Further edit: You DO NOT have to be a Costco Member to order from Costco Online. However, non-members incur a 5% surcharge on the prices shown on-site.
Forgot to say, they give you the option to have an online sub for 15 quid for the year and that means you don't pay the surcharge. On this item you would actually only be paying 6 quid extra with membership included. Worth thinking about.
Offer ends 8/01/17.