It really depends on what has been wrong with the phone. It's tricky forcing a replacement if it has been different issues that have been wrong with it. Not impossible though
If it's clear manufacturing defects, then most retailers use a kind of 'three strikes' rule before replacing goods, although this is usually their own policy, and not because they're legally required to.
I'd agree with contacting your Consumer Advice agency, or maybe the Consumer Direct website? They'll give you far better advice than a bunch of well-intentioned but sometimes misinformed AV geeks