You can buy a cheap TV aerial amplifier from Maplins, PCWorld, Woolies or any decent electrical dealer for around a tenner. Just plug the aerial in to the input and a lead to the TV from the output, might improve your reception. One thing to check on first, is to make sure all your plugs and sockets on the TV leads are nice and tight, no bad connections.
These digiboxes are a law unto themselves, some will work quite happily on next to no signal, whilst others need a powerful signal. Price means nothing, the best one I have bought is a Matsui, cheap and nasty, but it works ! The more expensive Philips and Samsung models ( in different rooms) are c**p in comparison. It's all a case of suck-it-and-see. Good luck.