Could not download all repository indexes

I've recently installed UBUNTU 15.04 beta to try it out

I get the folowing message when updating using synaptic.

"The repository may no longer be available or could not be contacted because of network problems. If available, an older version of the failed index will be used. Otherwise, the repository will be ignored. Check your network connection and ensure the repository address is correct in 'Repositories' under 'Settings'"

Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/source/Sources 404 Not Found
Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-amd64/Packages 404 Not Found
Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-i386/Packages 404 Not Found
Some index files failed to download. They have been ignored, or old ones used instead.