Dear Percy
As regards to your question as to whether you need a structural engineer, If you imagine a triangle, well the ceiling joist will act as a tie to stop the rafters spreading, as well as supporting the ceiling. However, this can be overcome by putting extra ties so you can have a clear ceiling line. Installing a Velux roof window, you will have to double up on your rafters to take the extra weight. Make sure you remove as much dust and debris as possible before laying it on top of the old ceiling, seal the door with masking tape so that no dust can spread to the rest of the house.
For any free, impartial advice or a quotation (if you do not want to do this work yourself) Call Alister Constructions and ask for James.
07393693455