I've a friend here in the U.S. that was unfortunate enough to purchase one of these. After having the machine chew up and spit out numerous small bits of the string and having the same difficulty you mention, he threw the whole thing in the trash bin and bought a different brand. He showed me some of the reviews he found (unfortunately, after buying the machine). It'd be nice to be more positive, but it looks like you may have the perfect excuse for getting rid of the menace...
Here's but a small example: http://www.amazon.co.uk/product-reviews/B000S0 KY94/ref=cm_cr_dp_all_helpful?ie=UTF8&coliid=& showViewpoints=1&colid=&sortBy=bySubmissionDat eDescending
Thanks Clanad. I thought it was just me, but having returned my first one to B&Q the replacement was exactly the same! Judging by other users' comments there must be a design fault with this model. I'm off to B&Q again at the weekend!!