Multiple accounts - addresses

[TC Man]

Hi!

I wished to change the primary email address for my Google+ account, but when I tried to add an alternative it said that the new email is already associated with a Google account.

This is one of my email addresses.

Then when I tried to sign-in using this alternative email address it said the account is "managed by Virginmedia", but gave no indication of how to proceed.

How can I replace the present primary Google a/c email with a "virginmedia.com" email?

TIA.