Oh well, that's another £20 quid down the drain.
I'd phone them and complain (edit: if you're sure it's wrong), they'll likely ask you to send it back for a refund and then, because it's their fault that you have to do that, insist they refund you the postage also.
I mentioned this as a by-the-way comment in a post a short time ago: They sent me a pushrod last year, along with some other bits, that they admittted, on the phone, they knew were made the wrong size and insisted I had to send it back to them for a refund. After I returned it I insisted they also refund the postage I had to pay to send back a piece of useless steel rod that they shouldn't have sent out in the first place. Although it wasn't a lot of money I sent it back purely out of principle and in the end they did refund me in full including the postage, it took several emails and about 3 weeks.