As I mentioned in some other thread, I bought one in Evans while I was up in Scotland. I had the same problem of it being a bit tight on my butt, but it's okay
(apple shape it very common in the UK and Ireland, it seems; big bottomed girls are not very contemplated!). Sadly, I didn't get to use it this year, as I was not able to go to the beach when I came back 
Hopefully it won't be too loose when my boyfriend visits next December!