Now normally I would say this was down to corroded terminals on the light units but I had both off recently and checked them and there was no corrosion ETC]
GUARANTEED SOLUTION HERE - NO PROBLEM AND COSTS NO MONEY. Spouse's clio exact same issue - you take the rear light cluster apart just as if for a bulb, but pull out the electrical connector. Ccontact 'prongs' sticking out of the connector for the indicator/brake, I think it is the third one down, but you can do the fix for all the prongs.
TWIST THE PRONG(S) a little IN THE CONNECTOR then they make proper contact - DONE!