I think Disco Fever is correct . I had similar problems using an existing aerial and an SLX booster.
Aerial direct to tv was ok but via the booster BBC2 was unwatchable on analogue and a lot of digital channels esp. Fiver and Film4 weren't very good.
Fitted a new wideband aerial and the digital pics via the booster are better than without and the analogue reception is also very good . Try fitting a wideband aerial, one with the digital tick on it.
This is a similar 14 element aerial to the one I fitted but it came with a short mast and fixing bracket I don't think this has one of those.
http://www.argos.co.uk/static/Product/partNumb er/5356443/Trail/searchtext%3ETV+AERIAL.htm