If you're currently getting excellent reception on analogue TV, with your portable aerial, start by simply giving it a try. If (because you're really close to the transmitter) your Freeview box can 'lock onto' all available channels, without any signal break up, then you clearly don't need to do anything else.
If your portable aerial gives you quite good reception, but with some missing channels or picture break up, you might get away with simply adding an aerial amplifier, such as this one:
http://www.maplin.co....gle&u=223504&t=module
Otherwise you need to get an aerial fitter to provide you with something like this on your roof:
http://www.maplin.co....eband-tv-aerial-22523
Chris