Well, received the case today - unfortunately it's big enough that my 505 fits snugly into it! It's too bad, as it's nicely made - if it were only an inch or so shorter.

However, it'd be a good alternative for the 650, if anyone prefers a wallet to a cover.
I think I'm going to have to break down and purchase one of the Sony 350 covers... bah humbug.
I may put the wallet on the marketplace here - someone might find it useful!
Edit: Just phoned my local Waterstones - they don't think they'll be stocking any covers for the 350 at all! o.O How odd - and there's a 2-4 week delivery delay when ordering online from either Waterstones or Sony UK. Looks like I'll be using the wallet for a wee while...
Edit 2: Okay, I'm an idiot.. just checked the packaging, it's the wallet for a 600!! I've contacted the seller and hopefully will get it exchanged for the correct one!