Apple ID - cannot reset mac, asking for old email password

So I am trying to reset my mac.

Firstly it asked for my current apple id and password, all good. Then it asks me to sign out of Find My Mac - here it presents an old email address and asks for the password. I have tried all combinations of password to no avail. When I try and retrieve/reset the password for the old email address it says that it is 'not a valid Apple ID' - so I'm stuck!

I never thought I had any duplicate apple id but I have used the old email address in the past as my main apple email address.

Any ideas?