You are unlikely to need a new TV. While more expensive TVs can have better electronics, the great advantage of digital (new method of transmitting TV) over analogue (old standard) is that as long as the electronics can tell the difference between 'on' and 'off', you will get a perfect picture.

Amplified set top aerials simply amplify the signal and the noise (unwanted signal). External aerials catch more of the signal, so you are more likely to get a useable signal.

The aerial I bought from Maplin was part no. A20GY for £29.99. The balans is already wired up. Simply take this and plug it into your TV - leave the long bit, whiskers (elements) and mounting bracket in the box. I think they have a 30 day money back guarantee, so nothing to lose (Please check first).