Unable to validate Serial Number installing Lightroom 6

I've just bought, and paid for lightroom 6, standalone version. I've logged in to my adobe account and gone to my orders and found the download option and downloaded the file, hit install. It then asked to login to adobe for my adobe id which i did, that was fine, and then enter the serial number. I entered the serial number provided in my download order but it won't validate it? Entered it a few times now and no good, I get message that 'We are unable to validate this serial number for Adobe Lightroom.

So how can i install it? Its the right number. I did have lightroom 5 installed so thought that maybe the issue so removed that, having made backup of catalog files, but still won't validate and install.

Installing it on a MAC if it makes any difference

Please help?