A signal amplifier might help but, in general (& assuming that you're in at least a fringe Freeview reception area), the most important thing for good reception is a decent aerial. The one which has been repeatedly recommended by ABers (and subsequently praised by everbody who's bought one) is the 48-element model sold by Maplins. The catalogue reference is PT18U and it's priced at �29.99 (plus, of course, if you're not skilled at DIY while up a ladder, you'll need to pay someone to fit it).
You can find your nearest Maplin store here:
http://www.maplin.co.uk/StoreLocator.aspx?doy=13m2
or you can buy online here:
http://www.maplin.co.uk/Search.aspx?criteria=PT18U&d oy=13m2&source=15
Chris