Reinstallation issue

I have a late 2011 iMac that I'm cleaning up for a sale. I have erased the hard drive, however when I try and reinstall the OS X 'el Capitan' I receive the following message. "The item is temporary available, please try again later" I've tried for a few days now and Apple are reporting no issues, so I'm guessing there must be some kind of conflict somewhere.

I've read that it could be the clock is incorrect, however without an OS I cannot see how to amend, if this is the issue.

The iMac is from the UK however I'm currently in Spain, so I'm redirected to the UK store where I have my account registered.

Any help that can avoid a mission to the Apple Store would be appreciated : )