Thanks for your reply.
We have the TP-Link TL-WR802N – 300Mbps Wireless Nano Router.
We took it downstairs, reset it and connected it to our main router with an Ethernet cable. We managed to connect to it on our mobile phones.
Taking it back upstairs, we connected it to the YouView box and the LED stopped flashing, which we assumed meant it had connected.
However, when I went into the settings and entered the IP details, it still said it couldn’t connect to the internet.