if you really want to sort them out, you can take them to the small claims court, as goods sold, even privatly have to be "as described and fit for purpose", tell them this and write a letter stating unless you recive a full refund in 14 days you will be forced to take action against them. See how that goes, then if you really have to you can take them to the small claims court under the trade descriptions act. if you ring your local trading standards or citizens advice centre, they should be able to help you. But to be honest a polite, firm letter threatening action will probably do the trick, as they won't want the hassle of going to court
DONT LET THE FUCKERS GET AWAY WITH IT!!! PLEASE!!! make sure you get your money back or the lights replaced