This seems to be the cause of my problems. I have changed my settings to GSM only and it has fixed my issues. I have a mate who lives in the countryside and he has had the same problems, and this was a fix for him.
Yeah, you get slower data, and I have noticed a difference in call quality, but it's infinitely better than no call at all! This is only an issue in my house, so the rest of the time I put the original setting back on. You can get an app which changes your settings based on your location. I think it might work for these settings, but I am not 100%. The app is called "Locale" and costs around