If you paid by Credit card and it was over £100 you can claim the cost of the goods back from the Credit Card Company, they are equally liable under Section 75 of the Consumer Credit Act. Free and much easier than Small Claims Court, best part about it, if you paid for the item, worth £5000 with £4999 cash, and £1 off your credit card, the Credit card company is liable for the full £5000 under Section 75.
Also if its under £100, and you used a Credit Card, try to get a chargeback made, its often quicker, and again free, rather than forking out for a claim at small claims court.
You can also tell the Trader that you'll be starting to charge for the storage of the goods they sent that you didn't order, do it in writing, recorded/special delivery, and make sure the charges are exorbitant, then claim that back from the trader in Small Claims Court, works out even better as they have to pay the disputed amount into the Court or be in contempt, and as its your local Court they have to attend, even better if they are the other side of the country and don't bother to turn up as the judgement will go in your favour automatically, and they will be charged costs by the Court.
When people start being unreasonable, its time to fight back.