Is there a foolproof method for getting threline straight where the ceiling meets the wall and they are different colours - tried masking tape etc

A painters fool proof method is a keen eye. Paint the ceiling first and allow just abit of ceiling paint to come over on the wall. Cut the wall paint up to the ceiling with a wet edge of an angled trim brush using a long firm stroke. There are edging tools available that have small wheels on the edge of a paint pad that actuallly do a pretty fair job.