The coach has come together nicely, what have you based the roof detail on?

The roof detail is based upon the drawing in the January 1981 Railway Modeller.  (I'm not entirely sure it is completely correct in terms of number of ventilators as I was trying to match details shown on the side elevations with that shown on the ends).  

The rainstrips are 0.64mm plastic strip, glued in the middle and at the ends and then allowed to take up the natural curve of the material.  The large ventilators are three pieces of 0.5mm sheet laminated together, then wrapped around with paper.  Finally a 2mm piece of plastic sprue is laid down on top.  The small ventilators are more plastic sprue, this time without the large bases and stood up on end.  

It's not entirely right, but it's not a million miles away either, and looks convincing enough from about 2' away. 

Thanks, the real thing seem remarkably camera shy

Most of the photos I have of them, they appear in the background of something else and are therefore hazy, small and far away, out of focus, or otherwise not of much use.  The only good clear photos I have seen are those in George Dow's GCR books.