reviews extend past browser width

Have just dropped the shortcode into a blank page and the reviews area is wiiiiiide, way wider than the browser width.

It's supposed to be a 3 column single row layout using the Advanced slideshow settings.

I'm using it in a custom theme, and have tried deactivating all other plugins, with no success, no errors showing in the console.

Presume this is conflicting with some CSS somewhere, but thought I'd check whether this is a known issue before disappearing down the rabbit hole to fix it.

Screenshot: