This is verging on the ridiculous now, and getting very tedious.
If a/c are on own nav, of course radar separation continues to apply, as it does if they are on radar headings... it's all about the degree of monitoring. If a pilot is on his own nav, he can turn to avoid weather etc (by the way - route direct to you may mean one thing, but to a pilot, following company policy it may mean another - they may have to go via certain waypoints etc (this is stretching it a bit far I know but when we are talking about pilots with different first languages, continue heading or fly heading is unambiguous))... if he has been instructed to fly a heading, he is more likely to ask before he turns - it's basic human nature.
and before more pedantry prevails regarding the fact that the pilot should always ask before making a heading change in CAS. we know this and pilots know this, but the fact of the matter remains; if a pilot is on his own nav, he is more likely to deviate first then ask later than if he had an assigned RADAR HEADING.
and just to cover my back a bit better (and this does not apply to you DFC - you, at least, sound like you have a modicum of common sense), most pilots will understand without having to be told in the most minute detail, that if they are on a radar heading, it is for a reason and will therefore have a bit more savvy about them regarding turning on their own whim.
