As others have indicated, the signal strength is insufficient. (Different makes and models of TVs have vastly different 'sensitivities', i.e. their ability to cope with weak signals. Your new TV clearly has poorer sensitivity than whatever you had before).
Ideally you should get your aerial (AND the associated cabling) replaced but, if you've previously had good Freeview reception with another set (or set-top box) then a simple signal amplifier will probably fix the problem. They're cheap in Tesco and Asda but it's worth paying a bit more for the one that Maplin sells because it provides greater signal gain:
http://www.maplin.co....gle&u=223504&t=module
Chris
(PS: Signal conditions were particularly poor over the Christmas period. That was probably due to the atmospheric pressure and possibly because of a solar flare, which are among the many factors which can effect TV reception).