Make sure you have all of your correspondence (email confirmation of purchase, etc...) and then send copies of everything to their customer support department with a strongly worded letter including the exact dates of purchase, calls/visits to follow up, who you spoke to, etc... The more details the better. Keep a copy of this letter as well. You may have to send more than one and if you can email do that too, again keeping a copy in your sent folder.
Going to the Office of Fair Trade is a lengthy affair and although it should be considered, you probably only want to try it after you've exhausted all avenues of resolving the issue with the company directly.
However, threatening to take it to the OFT is never a bad idea, especially if the company knows you have all your paperwork and dates in order and can prove your case!