Download Hiren boot disc, you'll need another computer to create a bootable USB. Boot to hirens and use lazesoft tool and enable the built-in admin account and grant your other account admin rights.
Thank you but I have now found a simpler solution.
1. Execute a safe boot with command line access;
2. Login to built in Administrator account;
3. At command prompt execute command "net localgroup administrators MyAdminAccount /add";
4. Then restart and elevated rights have been restored to MyAdminAccount