As others have indicated, your aerial isn't good enough. (Digital TV is broadcast on different frequencies to analogue TV. Some aerials can handle those new frequencies well, whereas others can't).
Theoretically you should get an aerial fitter to change both your aerial and the associated cabling, but you will probably get sufficient improvement in signal strength by simply fitting an aerial booster. Asda will sell you a reasonably good one for £9 but, for maximum 'gain', I'd recommend spending slightly more at your local Maplin store (or online):
http://www.maplin.co....gle&u=223504&t=module
Chris