(2-part post):
Firstly, use the postcode checker to see whether you're meant to be able to get digital TV yet:
http://www.freeview.co.uk/availability
(Also ask your neighbours about their experiences of receiving digital TV).
Assuming that you can, you need to be aware of three things:
(i) when the changeover to digital was first planned it was thought that up to a third of all homes would need to improve their aerials to ensure good reception. The actual number of homes needing a new aerial has turned out to be far less but you still shouldn't be surprised if your old aerial isn't good enough ;
(ii) in areas where both analogue and digital signals are currently available the digital transmitters are currently operating at reduced power. This is necessary to avoid interference with the analogue signals. As soon as an analogue transmitter is switched off the digital transmitter for that area will start operating at full power ;
(iii) devices which receive radio signals (whether they be radios or any type of TV) have varying 'sensitivities' (i.e. their ability to 'pull in' weak signals'). Some makes or models of digital TV will have better sensitivities than others. Yours might have a fairly low sensitivity, requiring a better aerial.