Can't speak for your exact case - That being said, every phone I have ever purchased has been from CEX (£300-£600) and the one time I had an issue, it was replaced by CEX within a week of my complaint, no questions asked.
Their B products are almost mint condition as well, if you're looking to save a few quid that way.
Never, to be honest I hadn't even considered that. Their warranty would likely cover that. I think it's 2-3 years default?