Ok chaps. Have done a "search" and having spent a good half hour trying to find the answer to my question I feel the need to elaborate...
Aircraft was given instruction "direct to XXX then YYY, report your heading when established". Heading was read back. Nothing further was communicated by ATC. There was then some confusion amongst the crew as to whether they were on a "radar heading". Approaching XXX, the crew queried this and were cleared direct YYY.
The answer to the confusion may (with hindsight) be obvious but the fact that there was confusion on the flight deck prompted the original question.