Arran, the Mib_Amb_Occlusion node does not need photons or a light source to work, You don't even need to adjust the occlusion attributes for it to show. Those atttributs control how it looks, not for making it show up, and Judging from your render, it looks like you didn't plug your occlusion node in correctly.

Double check your connections. Make shure you don't have it plugged into your materials common attributes like color, trasparency, ambient, ect, if thats what you have done. It has to be used in your materials shading group, in the mentalray section. The Mib_Amb_Occlusion is a mental ray node, hence the "Mib"

Try it again and let me know if the problem persists. Perhaps it's a technical problem.

EDIT: You don't need Final Gather on for it to work either. FG has no effect on Mib_Amb_Occlusion.