Distribution amps are designed to take a signal (or sometimes more) and feed to more than one source, they not only amplify the source but also isolate it so one TV can not interfere with another.
The problem you have atm is that a standard Y adaptor will join both inputs thus allowing one signal to run to the source of the other, in other words it does not isolate the outputs. Also this will reduce the signal strength. You may be able to get away with a cheap Isolating Y splitter but as you add more and more TV's the signal will drop hence the suggestion of a distribution panel (If you can't find a two input one then you'll need a separate one for the Sky and the Freeview.)
Something like this is perfect for you (though slightly over spec as it will allow you to use more than one sky box in the future)
http://cgi.ebay.co.uk/Labgear-HDU681...item4cf746a777