Lightmapping artefacts

Hello folks,

I've got a very low poly rock face which I've made in Blender (so no overlapping UVs here thank you very much):



I'm trying to add this to a scene and bake the lighting however the result is producing a lot of dark artefacts along the hard edges of the mesh, like so:



Here are my lightmapper settings:



What should I be tweaking to reduce the artefacts along the edges of the mesh so everything looks neater?

Thanks!