02 is shit

I bought a Palm Pre about 5 month ago. After some time the speaker sometimes stopped working (about once a week) so I decided to get it fixed. I sent it in and after a few weeks I go a phone call telling me they can not fix the speaker because of a crack in the USB port and the phone was sent back. As soon as I switched it on some testing screen appeared and the phone is now utterly useless as the only thing I see is a system test running and there is no way to get into the actual phone any more. So back to the shop and they refused to take the phone back as it just came back from repairs. So I called the help line and they then said I should send it back. Again 3 weeks later I got a call saying they can not do anything because of the broken USB port and after some discussion the crack was actually a scratch near to the port in the plastic. And the records show that they have never touched the phone. This is a lie as I have no idea how to get into this testing program. So I tried to dispute this which is only possible with the shop. After doing this and waiting another month the phone came back untouched and is still showing a white test screen.

A wrap up: I sent in a phone because of a little problem and I get a broken phone back which they now refuse to repair. So O2 broke my phone and they get away with it.

Some ideas what I can do?