You've not said which digital service you're using but, since you refer to channel 105, I'll assume that you're referring to using a Sky box or similar equipment (rather than receiving terrestrial Freeview, where Channel 5 is, logically, on channel 5).
Further, I'm assuming that you've not got a Sky subscription, so you're only receiving the free channels.
There are two types of free satellite channels. Most are FTA ('free to air') services. They're not encrypted, so they can be received by any Sky box even without a valid viewing card.
However a few channels (notably the 'Five' group, such as Five and Fiver) are FTV ('free to view') services. Such services are encrypted. While you don't need a subscription to watch them, you do need a valid viewing card. You can order a card, for a one-off payment of �19.57, from Sky by phoning 08448 244 400.
Chris