Installing El Capitan on a 2007 iMac

I have just wiped an old iMac from option R - install OSX without knowing that El Capitan [the last available system for this model] is unavailable in the App Store. It has gone though the motions of downloading it but now says there is no packets were eligible to install. Every time I restart now get the same thing so can't use it at all [images attached]

I have downloaded El Capitan from here to a drive [https://support.apple.com/en-us/HT206886] but I have no idea how to access it when I cant do anything on the iMac.

Any help appreciated.