This is covered under Consumer Law absolutely.
Whether you had worn it or not is irrelevant- you don't expect to pay that amount for a jumper for it to be unusable so quickly. Had it been a cheap £6 jumper, then yes, fair enough, but not something you have paid £90 for.
Try writing something like:
This is covered under ‘consumer protection’ which states ‘it is the policy of current legislation to protect consumers against unfair contract terms. In particular, they are protected against terms that attempt to exclude or restrict the sellers implied undertakings that he has a right to sell the goods, that the goods conform with either description or sample and that they are of satisfactory quality and fit for their particular purpose’
In this particular case, the items are not of satisfactory quality or fit for their particular purpose.
It is not of satisfactory quality in my opinion for this to be unwearable after a couple of months. And there is no time limit... it is just a reasonable amount of time (as per the example above, you wouldn't expect a £6 jumper to last a year, however, a £90 jumper I would say at least 18 months with very regular wear).
(I am not legally trained, however, I am a law student and studied this is my first 4 years).