You need to improve your aerial system.
Digital channels are broadcast in 'bundles' (called 'multiplexes), with each 'bundle' using a different frequency. In a perfect world you should have a different aerial for every frequency used (which would mean dozens of aerials on every roof!). That's clearly impractical, so all aerials only provide a 'rough match' to the incoming frequencies. Your aerial is good enough at pulling in the signals on certain frequencies but not on others (so it's only providing your TV with strong enough signals for one or two 'bundles' of channels).
If you were able to receive some channels intermittently (or with occasional picture break up) I'd suggest trying a cheap signal booster but, since most of your channels are missing altogether, you also certainly need a better aerial.
Look for an aerial fitter with the 'CAI Plus' logo and get him to fit something like this
http://www.maplin.co....eband-tv-aerial-22523
or this
http://www.maplin.co....ital-tv-aerial-221124
(A decent installer will probably change the cable at the same time)
Once the new aerial has been installed you'll need to retune your TV. (If there's a choice between 'adding' or 'replacing' channels use the 'replace' option. That forces your TV to start completely afresh).
Chris