Minimum radar separation range of a particular radar source depends on the azimuth error distribution of the radar source. One radar may be good for 3nm at 50 miles, another 3nm at only 40nm (for example). Each radar source (well, NATS one anyway) has a published separation range for 3nm, 5nm and 10nm, where appropriate. I wasn't aware of general figures being used, unless in a unit's MATS Pt2(?).
It also depends if you are separating SSR from SSR, SSR from PSR and (worst case) PSR from PSR.
Then there's MRT........
RS