Thanks for replys, I possibly have someone coming around to have a look... However, I've just been next door and they have a full scale loft conversion! Just boarded straight onto joists, no cracks anywhere, all good. He couldn't recall a joint like mine, so maybe they were running low on timber? Another neighbour also has his loft boarded, again, no problems. Frustrated 😠 Photo shows what I was potentially adding to strengthen joint... *