The suggestion that elchasto made works! I didn't even have to use the coupon (although I tried earlier, so that may have been a factor). I just added it to the "cart" and the amount owed was zero.
Interesting times we live in!
Well I found out that sometimes it works after a few mins
Using the add to cart method is just the way after the few mins
I don't know if this would consistently worked, but it just worked for me:
Add to cart
Log out
Log in
Check out
I guess it must be noted repeatedly here that they've changed their interface and that 100% free courses no longer have the 'add to cart' option at all.