Television transmitters (both analogue and digital) are designed to provide a usable signal, throughout their intended reception area, for viewers using a good quality, outdoor, unidirectional aerial. While people living close to a transmitter can sometimes get a reasonable signal with an indoor, multi-directional area this should be regarded as the exception rather than the norm. i.e. manufacturers who suggest that most people will be able to watch TV by simply plugging in a DVB-T device (or similar) are misleading their potential customers.
As has been suggested, you probably need to use an external aerial. If you've not already got a suitable aerial, the 48-element PT18U, from Maplin Electronics, has been well-reviewed (and frequently recommended here on AB). Unfortunately, the Maplin web page is currently malfunctioning, so I can't provide a link. (The last time I saw it advertised online, the price was �29.99).
If you're unable to use an outdoor aerial, you might get sufficient signal strength by using a decent indoor aerial which incorporates a built-in amplifier. (Maplin sell a reasonable one for �19.99).
Maplin Electronics is one of my favourite stores because (unlike Curry's, etc), their staff usually know what they're talking about (or will call upon the knowledge of a more experienced colleague when they don't). Call in at your local branch and they should be able to help you.
Chris
(PS: It's worth remembering that, in areas which haven't yet switched off the analogue transmitters, digital services are currently running at reduced power in order to avoid interference)