Give this a try:
1, From your desktop, select Go from the top toolbar
2, Press and hold the Option key on your keyboard **This is really important - it allows you to access folders on your Mac's Hard Drive rather than your user folder**
3, Select Library
4, Open the Application Support folder
5, Find the Sky Go folder
6, Move everything IN the Sky Go folder to your trash. **Don't take the enclosing folder, just its contents** - anecdotally, incase it helps somone identify the contents, the first folder/file in this folder for me is called "blob_storage" and the last is "WebStorage"
7, No need to empty your trash bin, just restart your Mac
When you next open Sky Go you should be given the option to select cookies and then log in.