I thought it might be the CPU getting warm, but I think the battery is on the left on every recent model of iPhone, so either it's getting hot or swelling against the screen, which is even worse.
Good shout, I hadn’t thought of it potentially being a heat issue. It worked perfectly before, even with the smashed screen. I’ve sent a fairly pissed off email saying I want the phone replaced, we’ll see how that goes.
I thought it might be the CPU getting warm, but I think the battery is on the left on every recent model of iPhone, so either it's getting hot or swelling against the screen, which is even worse.
(Or it's just a shite screen)
Get the whole thing replaced.