There are two types of free satellite channels.
'Free to air' (FTA) channels aren't encrypted, so they can be received by anyone who's got a satellite dish and receiver. The majority of free satellite channels are FTA.
'Free to view' (FTV) channels are encrypted, so they can only be received using a satellite receiver with a valid viewing card to decrypt the signals. Channnel 5 and its sister stations are FTV.
It seems that you're only getting the FTA channels, which suggests that you've not got a valid viewing card. Sky will sell you one for a one-off payment of �20. (i.e. there's no subscription and the card never expires). Phone 08442 410 595 to purchase one.
Chris