Ok, I have (kind of) solved this so will post a solution for anyone who may stumble upon this thread.....it's a bit of a work-around that addresses the symptom not the cause....
Try what I may I couldn't set the style, but given the text always followed the format of:
some text in normal style, carriage return, paragraph mark, and some more text in heading 3 style
I solved this by adding another CR and PM before the first CR, then deleting the CR and PM that had the incorrect style.
The new CR and PM automatically pick up the correct style from the first block of text. Solved!