First of all under Englishj law your beef is with the person who sold you the box NOT Thonmson. (Its up to the retailer to make their own arrangements to get their money back.)
I would suggest that you go back to the RETAILER who sold you the box and and say that 18 months use from a STB is not acceptable. Huge numbers of Set Top Boxes last years and so because the box that you have been supplied with has only lasted 18months: "Is not of merchantable quality" and you "Reject" it and would like a refund. The one year warranty is over and above your rights in law but a lot of people will try to restrict you to it - they are wrong. Stick to your guns and demand a refund.
Terry
April 2006