Glreadpixels equivalent (colour and DEPTH)

I have seen code relating to Viewport->ReadPixels to capture colour bytes.

But I also need to capture the corresponding depth values (of the whole final scene) for further processing outside of the shader system.

So I need the UE4 equivalent of a glreadpixels call.

I don’t mind if the depthbuffer resolution is 24bit (as I believe that is what most cards default too) but if there is a 32 bit mode than that’s a bonus.

Does anyone know how to access the depth buffer in system memory?

Cheers!