Darren....
The clx works well with the superlight brush. Anything heavier makes it more flexible top end.
I've tried a vikan 11 oval, vikan sill, brodex light weight and a mono superlight on my clx.
The superlight brush gets my vote for the clx.
On first cleans I angle the brush and give the sills a good scrub. After this you can just sweep over the sill at your normal angle.