This may sound like a really stupid question, but have you paid up the £10 yet? If not, don't. Let them waste their time chasing you, and use someone more reliable in future.
If you have already paid, ring them and complain. Have your email visible to you when you are on the phone, as well as their reply, and most importantly of all: keep your temper, don't shout and definitely don't swear. Do all you can to maintain the high ground of a wronged customer.