First I would like to say thank you to all who gave advice. I now have had the Flightaware stick and filter installed, I've seen the maximum range go up from 100 to 150 miles, as a result I am also now picking up a lot more aircraft.

I'm now considering mounting the antenna externally, my set up is in the garage so I am undecided as to go for mounting on the garage or as high as I can get on the house roof. The house roof will mean a much longer cable run as the garage is detached.