Connect a laptop to your router with an Ethernet cable. Disable anything else which might be using your broadband (such as a Now TV box, or similar). Go here and check your download speed:
http://www.speedtest.net/
Then remove the Ethernet cable and repeat the test using a wireless connection.
If you get a pathetic speed in both cases you'll know that there's a problem with the phone line (or cable) to your home, which only Sky (or BT OpenReach) can fix. If you get a decent speed with an Ethernet cable, but a lousy one with a wireless connection, you'll know that it's a wi-fi problem (which might, for example, require you to use a different channel).
Let us know how you get on and we'll try to advise you further.