Fusion render node doesn't appear to use openCL

I have a scene with volume fog, supper fast using openCL. in gui Fusion. When I open GPUz I can see it using my GPU to process the volume fog.

However when I use the rendernode. on the same machine, I can't see it using any GPU. and takes much much longer. Both preferences for openCL are set the same. this is a major slowdown. Is this a known issue, and is there anyway to work around it?

Thanks
Chris