Phones 4 u refusing to exchange mobile phone for 2nd time

I purchased a Sony Ericsson C510 on the 22nd of May. The phone developed a fault on the 15th June and went to the shop and got an exchange for the phone, the same phone again. The fault was that the phone was saying that a Sony Ericsson battery wasn't in place and it had a permanent white screen. It also said that my sim card had no network coverage. So my phone was working OK until this morning when I went to use it and it had switched off and when I turned the phone back on it was saying that I had no battery and no network coverage. When I went to charge it it said it wasn't a Sony Ericsson battery. So I went to Phones 4 u as this is the 2nd problem with the same phone, and I spoke to their 'Customer Services' and I use that term lightly and they were rude and said under no circumstances was I entitled to a refund or exchange, only a repair. But this is the 2nd phone with the same fault and the 1st fault was irreperable. So I am a very angry customer who feels a bit cheated and I am waiting on a phonecall from a higher management. I am so confused as I have been told so many things. Help.