I'll be a little gentler than Skywalker; but what he says is essentially the truth.

I used to work for Clarks and their standard policy is that shoes are only guaranteed for up to 3 months unless otherwise stated. Basically, Clarks have no way of knowing if a customer is telling the truth about how often or how little they've worn their shoes, so they have to err on the side of caution and have a set procedure and policies in place for everyone. They cannot guarantee how the shoes have been stored (which can have an effect on deterioration) so it's unfair of you to expect a refund of exchange after 6 months of use. Add to this the fact that you probably won't have your receipt and therefore they can't prove how much you paid, or when you actually got them.

Some Clarks stores MAY give you an exchange under goodwill, but they're certainly not obliged to. By all means go and ask, but don't kick up a stink if you're refused as you're not legally entitled to one.