This site is good for checking what aerial you need...
http://www.wolfbane.com/cgi-bin/tvd.exe?DX=L&HT=10&OS=m43Try entering you full post code. If you just enter M43 it says...
There is a choice of transmitters and the aerial recommended is different depending on which one your should point at.
Winter Hill = Group CD, Horizontal polarisation
Saddleworth = Group B Vertical polarization.
Both Amplified extra high gain types. If your new aerial is a wideband that might be part of the problem. It could be picking up the other transmitter. (EDIT: A Group B aerial just covers part of the TV band but that doesn't mean fewer channels - all the digital channels on the Saddleworth transmitter are within the group B part of the TV band)
Check the polarization is right (elements vertical or horizontal).
Check the UHF output frequency on the sky box isn't similar to one of the multiplexes from the other local transmitter (or the analog channels).
Try plotting your position and the transmitters on a map. See if your aerial is pointing at both because of the way they line up.
Repeat the postcode lookup with it set to Distant at the top. Plot those transmitters as well and see if you are pointing at one of them as well as the correct local transmitter.