You are picking up the WiFi signal from a Sky router belonging to one of your neighbour's, as you have found, the signal is encrypted. There is no generic passcode for WiFi encryption, they are unique to the Network and current encryption methods are essentially uncrackable.
This is very lucky because should you try connect to your neighbour's network, you would be breaking the law. Many people have received some serious fines and had their computer equipment seized. You would also be very unneighbourly because you would be stealing (bandwidth) from you neigbour, not Sky.)
T, April 2009