cracks between ceiling and walls of new ensuite

Hi Jane, sounds to me like the problem is no scrim used at the edge joins when it was plastered? If the walls are of studwork construction then they have not been fixed to the adjoin wall very well as they have slight movement possibly, also timber tends to shrink when it comes from the merchants and into a house for any project as the environment is centrally heated and dries out the studs etc so it can cause shrinkage cracks. Nothing to worry about unduly just wait a while before caulking and filling before decorating, maybe a top quality silicone mastic to the corner of the tiles to tidy that area up. Hope this helps. Graham