Well, I've had a good blow on it and haven't had any problems with it. I've had a word with the seller and he said he's never noticed the imperfection in it, he hasn't had it modded, and he got it brand new. He's offered full refund and postage back to him. I'm thinking at £40 I can't really be bothered as it plays fine.
I've sent a message off to Vandoren via their website and asked them about it. Let's see if I get a response. For anyone wanting to use their online form, note that it doesn't like .co.uk email addresses. It took me a while to work that one out as I just kept getting an "error in address" message in French whenever I hit the submit button. I filled in all the parts it wanted for a postal address, removed spaces, put in a phone number, used a different browser...
And a final thought, surely mouthpiece manufacturers test play all of their mouthpieces before boxing them up and shipping them off?