alphaalpha - interesting debate you started. I take a slightly different view from that of my esteemed ATCO colleagues.
As has been mentioned, the Manual of Air Traffic Services states that aircraft operating inside controlled airspace are deemed to be separated from unknown aircraft flying in adjoining uncontrolled airspace.
If the traffic outside controlled airspace is known to the controller and both aircraft are in receipt of a radar separation service i.e radar control and radar advisory then, in my opinion, the deemed separation becomes invalid and I would ensure that the aircraft were separated by the radar separation standard in use.
I don`t expect anyone to agree with me but that is the way I have always played it.