How can we improve out TV signal?

We live in a basement flat of an old Edwardian house and out TV signal is rubbish. We have freeview built into our HD tv and when we first moved in we got all channels with a really good picture. However about 1 month ago all the channels suddenly lost their signal and we only get channels 1-4 on terrestrial tv with a really poor picture and 6 freeview channels with a clear picture, but they are rubbish channels- BBC 1 and 2 which we have on terrestrial, BBC 3, DAVE, CBBC, BBC News 24. As you can see these are hardly the most interesting channels! We can't get sky as we don't own the flat and are moving into our own home in September so paying for installation etc would be silly. If there anything we can add to our TV, to the wall?? Our ariel is our landlords one who live above us and their picture is fine and they get all channels. The TV isn't broken as we took it to our friends house and it worked fine. Please anyone help, I'm going crazy with no channel 5 for neighbours! Thanks