Damp O/S front end

I was after some general advice on some damp ive become aware of, I suspect it could need the front of the van removing but thought id just ask for some general advice first.

The Van Is a Bailey Ranger S5 2007/2008. Just out of warranty by about 6months.! The area in question is the offside front above and behind the light on the front to the side of the van. This is the ranger with a single large front window.

I had noticed a little mould on the front wooden flat panel just beneath the lower offside blind runner for the front one piece window. I have also noticed that in heavy rain the water is bypassing the window rubber seal towards the top of the window at the side and flowing down the inside of the seal and collecting in the bottom of the window seal.  

Upon inspection in the front locker I noticed water has been trickling down the wooden frame that is behind the front GRP panel at the side of the window.

Hoping that this could just be the front side light leaking on the front panel I have removed the light cluster to inspect..... its worse, the wooden panel behind the light is rotting on the window side of the frame not the side that attaches to the side of the van,

The centre of the wooden frame out to the side of the van is good and solid however the side that is on the window side is soft.

This is also above the light fixing so is entering above the light area so I ask, is it entering on the roof seal where the front panel joins the roof or is it getting in through the window seals that are not sealed to the wood correctly.

I shall probably seek professional help but wondered if anyone has had this before or knows of anyone who has.  

Help!