Posted: 01/06/2014 at 12:09
Giving advice on ventilation depends on where it's sited, but it is rather low in the container which won't be helping. You can improve drainage by adding horticultural grit to the soil, so you could help with both things by gently digging the plant up (trying not to damage any roots), then mix enough grit in with the compost in the container so that the soil height is increased to about an inch below the top of the sides. Then re-plant it and put the slate back on top. The plant will then get a bit more air.
If you do that, it might be worth removing all of the soil from the container befor you mix in the grit and making sure there are plenty of drainage holes drilled into the bottom - those are absolutely crucial.