Does anybody know how to get a PrettyPhoto modal window to show up in the centre of the screen instead of the centre of the page?

I’m having some trouble with PrettyPhoto (within WooCommerce). When I click to show the image in a modal window, it appears in the centre of the page instead of the centre of the screen.

This means that on long pages, it is there, but I have to scroll down to see it, so those using my site who don’t know that end up thinking nothing is happening.

I thought it would be a case of writing some CSS, but either it doesn’t work or I can’t figure out the correct CSS element to use.