I discovered that one rear door seal had started to tear. The seal has a main rubber and an auxilary thin piece attached which is angled out and fills the gap to the outside. This thin flap started to tear off from the bottom, at the level of the window and became detached from the main seal for about 120mm.
I looked up the UK forum and this problem was spoken about. The new complete seal is expensive. Some people said some water runs in the door if this flap is missing but I did not notice that.
So I tried to glue it back together with black Permatex sealant. Using a subtle amount it was not strong enough. However I have now applied a more substantial amount and more noticable (if you look) and it seems to be OK. I will reinforce the other side which seems OK so far but prevention is better than a cure.
I believe the problem is most likely caused but the younger persons who use the window surround to 'handle' the door roughly instead of just pushing the outside gently shut. I wonder it other models, eg Clio, use the same seal design and same problem.
Cheers Jaahn