I second WebdyW's reply, but I would start by taking in the item(s) you have problems with, together with the assembly instructions, and ask them to take you right through the assembly process - behave modestly and politely but not meekly. If you have already done this (a calm approach is much more likely to give the vendor's staff a way to back-track, wade in too agressively and you are likely to harden their stance against you) then you are entitled to do exactly as WendyW suggests, but leave the "fuss" out of it because there is (in my view) no benefit in making a scene. You can make your point perfectly adequately in a calm way, and this is often the much more intimidating (and effective) way to put your argument forward. Before making an official complaint (via court or similar) speak to Citizens' Advice, they can arm you with jargon and legal clauses to quote at your opponent. If you paid for your item(s) with a credit card, you may have an immediate claim for a refund via your card issuer (depending on card, value of purchase, time elapsed, etc.). Good luck.