Hi All,
Thanks for the responses. I think I got to the bottom of it. It came down to my room modelling. I originally modelled a hollow solid with a rectangle window cut out. This initially worked ok but when I extracted the floor, walls and ceiling, the results were blown. So I remodelled an HB room with no wall thickness and an HB window attached. This worked fine.
So it seems like the problem was light leaks. A schoolboy mistake but goes to show the importance of getting the basic modelling right.