Boy, has this issue been bashed around or what.
A heading is a heading is a heading. No differencec, just some ATC's are more pedantic than others and insist that there is a difference. I never use the term RADAR heading as I would have already radar identified the traffic and any use of headings thereafter can only be RADAR headings, what other means or determination is there other than utlilizing the radar to determine correct use of headings.
To summarize:
If you instruct an a/c to fly a heading then the expectation is for them to follow that instruction and to advise ATC if they are unable.
The a/c will then remain on that heading until advised, i.e. 'locked' on that heading.
