I know some users doubt the validity of legal advice handed out here, but I am about to point you in the direction of a section of an act of parliament so you can check it out if you like. s.14 of the Sale and Supply of Goods Act is about good not being fit for their purpose. (Specifically s.14(B)). A good trick might be to mention the Sale and Supply of Goods Act next time you take it up with them. Also use the phrase "I have taken initial legal advice on this matter" - it often sorts things out VERY QUICKLY. It does seem to me that if you can't see through your glasses then they are not fit for their purpose - and thus you can envoke those statutory rights that everyone always wonders about when they see the words "This does not affect your statutory rights".
http://www.consumerdirect.gov.uk/ - this website will hopefully clarify.
Good luck!



