Although the old router could be used as a Network Switch / Wireless Access Point, it would still need to be connected via one of its LAN ports to the main router by ethernet cable.
What you need is a "Wireless Network Bridge". This is the opposite of a Wireless Access Point in that it allows a "wired" network to connect to a wireless one. You would plug your Sky HD box and Blu Ray player into the Bridge which would connect wirelessly to the router.
Examples click here (but avoid a Belkin Gaming Adapter as this only supports the connection of a single device).