Blank checkout page on marketplace - everything else working!

If I still had hair, I'd be pulling it out!

This is a product:
http://www.shensmithbarristers.co.uk/store/products/representation-at-a-small-claims-hearing

I can add it to cart fine - it is only set up to take manual payments, and I can see all the pages the plugin created. It is network enabled, and everything seems to be working until I get to the checkout, which gives me a blank page. What have I done wrong?