I bought one in LIDL some months back and it does an OK job. I would assume that if I paid more that I would get a better result. It plugs in to an electrical socket and has one co-ax in and 2 co-ax out ports.
I know Power City have them but don't know how good they are. Somewhere like Maplins or Peats could have a better range of choice.
My amplifier from Argos didn't solve my problems so I dis-assembled the cable again, shortened it and took more care when putting the connector again and most of the snow was gone.
Amplifying a signal that already suffers from interference will not solve the interference problem in case anybody was under any illusion. As mentioned above the best way to avoid interference is to (a) make sure that the original input is delivering a clear signal otherwise get ntl: out to fix it and (b) use good quality cables and connectors and reasonably short runs of cable to ensure no degradation in the signal.