downgrade from High Sierra to El Capitan

I run a Mac Pro 2010 with 1 SSD for Mac OS X and another for Windows 10. However, a faulty power supply unit caused corruption on both SSD's. This has mainly affected OS X High Sierra which will now suddenly make the computer reboot (with no warning) into Windows 10. If I get somebody to remove the Windows SSD, Mac OS X will run, only if I select it from the usual grey boot selection screen. If not, I get an error message on a black screen about no bootable or system drive.

In Windows, Mac Drive 10 shows major corruption on the OSX drive. However, I don't understand why High Sierra only has issues if my Win 10 SSD is in it's bay.

Due to the corruption, I have backed files up using Windows and want to wipe the Mac OS X SSD and go back to El Capitan. None of the stated start-up key combinations work (I wanted to reinstall via the Internet) and from Windows, I can't find a download for El Capitan with which to make a bootable USB drive.

If anybody could advise me, I would be extremely grateful.

Sarah