I don't understand the perceived condensation problem being an issue, however I think an insect screen would be fine. Can you remove the cap, put the screen on and replace the cap?

Thanks. If condensation wasn't an issue how would I close it off?. I don't think I can take the cap off as it's all one piece that slides inside the flue as far as I understand. Others have suggested replacing the current cap with a new one that has wire already around it abd adding mesh to that. To be honest I'm not sure how remove the cap in the first place though.

Hard to tell from the picture, but it could be the type of cap that twists on. That's how the various pieces of pipe go together, and it may explain why the cap is racked in one direction. If it doesn't come off that way, wrapping it would work.