Don't know about the Discovery one but this one might do you from Direct 4x4, although I don't know how much higher the safari roof is so might not fit if it's too high (see the listing)
It's the type I picked up for my Y61 and it is mahoosive! Very solid too, although there was a slight bit of damage to a small area of the mesh on mine (ordered mine through their main website and got it delivered to the farm but if its not through eBay you only have 24 hours from signing receipt to report damage so I'm scuppered :doh) but since I eventually plan to replace most of the mesh with something a bit more solid for my roof tent, I can live with it in the meantime!
Another one might be this, although it looks like you have to construct it so may not be as solid as the Direct 4x4 one?...
Its not so much the rack itself, its the feet, the only thing that seems to come close it for a high top transit, but they only seem to be 260mm, I have measured the gutter to the top of the roof, and its about 300mm so it needs to be higher than that.