Manufacturer's fault on phone

I have a Sony Xperia Z3 compact which I got in December 2014 through MTN on contract. In March it started turning itself off and we would eventually get it turned on again and then in May/June it started doing it more regularly. Then towards the end of June it turned off and we couldn't get it turned back on, so I took it in to our MTN store after being told by Sony to do so via email. MTN came back to me to say it's a manufacturers fault but due to the fact that it has a crack on it the warranty is void, so they want to charge us to fix it and they'll only give us a warranty of 3 months, so if it breaks again in 4 month we'll have to pay again. I think this is very unfair and I think Sony should at least help pay for it, if not fix it for free. If you Google the problem, it seems to be a worldwide problem that Sony has. Please can somebody help me with this.