Hi everyone I've managed to resolve this today the only way is to downgrade on smart switch on a pc. When you open smart switch then connect phone, you do have to wait a while for phone to be recognised before the option to downgrade will be shown. Also remember you'll need to swipe down on phone once connected to pc to select the file transfer option.
I've had to do it twice today before the second time it fully downgraded, the first time it got interrupted and said it couldn't do it so stick with it and hopefully it will work I did and it has. Good luck.