I would apply now. You can always reapply next summer anyway, it won't lessen your chances....on the contrary, it will show your determination.
You've got everything to win, and nothing to loose.
I know, they expect you to have a C-grade but as long as the rest of your application is good I wouldn't be surprised if you slip through. And as long as you can do basic math, and manage conversions with a calculator for an assessment day, you'll be fine.
Good luck :-)