iPhone 5s screen moving out & battery problems

I've got an iPhone 5s 32gb and developed a problem with the screen popping out slightly on the left hand upper side of the phone. When I first received the phone, the screen would slightly click on the left hand upper side.I contacted Apple & they recommended that I take it into an Apple store & get it tested for a battery expansion issue (Battery expanding & pushing out the screen).

The store tested the phone & found that the battery was fine. The screen was marginally unlevelled. Whilst giving no reason for this issue, they offered to replace my screen for £70, which I kindly refused. The problem was not big & besides they had not offered me a substantial reason for the cause of the problem or a direct solution.

Leaving somewhat bemused at the time, the issue has now exacerbated.The screen has slowly progressed in moving out to a stage where I can see a hint of the back-light through the gap between screen & body. The battery has lost a significant portion of it's life & regularly shuts down my phone (displaying charging sign) despite showing upwards of 30% charge. This fluctuates.

I contacted Apple via chat & they recommended that I get my Phone re-diagnosed. The notes from the former diagnosis did not include the exact cause of the problem (which is funnily annoying, what were they doing ? twiddling their thumbs or too lazy to type in actual notes).

Anyhow I visited an Apple authorized service centre on the advice of Apple customer care. The worker was also convinced that this was a battery expansion issue & noted that he'd come across it regularly. He done a quick on phone diagnosis & found the battery level was green & fully functional (which is weird, my batteries not working as normal). He also offered a solution to replacing my battery & screen for 140.

My phone's out of warranty and a simple check up will cost me money. I have never dropped it or damaged it. Neither Apple nor anyone else has offered me a real explanation to this problem. I really have no clue as to the cause & information on the web hints at different possibilities. I don't know how if it'd feel comfortable forking out cash if it's a problem with battery expansion. That's not my fault, it's a design flaw. If it's something else, I'd look into it.

Has anyone else had a problem like this or had to deal with Apple on an issue like this ?

Any supportive, constructive or helpful advice will be greatly appreciated.