Why not just ask for a free repair or replacement under the Sale Of
Goods Act 1979?  Go and ask for the manager, and if he doesn't satisfy
you then get him to phone from head office and see what they say.
Ultimately you might have to contact your local trading standards or
citizens advice bureau for help in taking the problem to a small claims
court.   Some people believe you have a one year guarantee - this is
nonsense, as is similar talk of warranties.  You can buy those if you
feel like it - indeed, many stores shove them in your face whenever you
try and buy anything from them as it's money for nothing - but you have
up to six years protection from manufacturing faults thanks to the Sale
Of Goods Act.  Obviously, the older a product is and the more wear and
tear its experienced, the more likely you're going to get just a repair
rather than a replacement, or perhaps not even a repair, but if the guy
in the shop had told you, when you were buying it, that after around 18
months or so it would stop working - would you still have bought it?

All this assumes you have some sort of proof of purchase, otherwise
you're probably screwed.

Hope this helps.

Not answering your question, but which lens?
I'm getting increasingly uncomfortable with the build quality of the
18-70mm (bought with a D70). It had to be repaired after the front of
the lens loosened up and could be wobbled by 4-5mm from side to side,
and now its started doing it again. I thought this was down to the
rather harsh use I give it - regular buffeting from 100mph winds when
pointed out of a 'plane, but someone recently told me their lens had
developed the same fault despite more normal treatment. Their repairer
was scathing about the internal design, claiming there were inherent
weaknesses in the engineering.

(I'll be away for a couple of days from lunchtime, so apologise for  any
delay in following up any replies.)