I'm trying out the settings recommended by the BBC and have found that the Knee setting in the picture profile is unavailable, grayed out
How do I activate this setting?

Thanks

The knee is grayed out in the following situations:
When the gamma is set to cine1-4, that meaning that it will only work with STD gamma.
when the shutter is on SLS or EX slow shutter is on it will also be grayed out.

it is all in the manual (at least the EX3).

Oh ok,
I do use the cine 3 and 4, this explaines it.

Cheers