How do we stop debris from outside coming through attic tiles and wooden lats in new build attic, without causing further problems with dry rot or damp? any thanks in advance.

You shouldnt have any problems, as you say it is a new build attic, I dont know if things are done differently in France, but normally there is felt laid prior to battens and tiles. However I would suggest what is known as torching, I dont think theres many trades doing this, as it was one of my first jobs about 40yr ago. Basically its a mix of sand, lime and horse hair, this is then used to point up the top halfs of the tiles.slates, sloped towards tops of batten. We used this method years ago on natural stone slates with out any problems, and still using this method on a lot of older properties. I wouldnt advise using the spray on foam insulation.

put in a breathable membrane over the beams then nail the lathes into place then put your roof tiles on finnish of with the ridges and cement the eaves. roof sealed but still able to breathe.