Thanks Ken and everyone else, that is some very helpful feedback and I will take it on board for next time. This was done using natural window light. I think next time I will turn the subject more to the light as you say. I think I might also get a bit closer for a tighter crop but leave some space above the head. Indeed, I don't think I would use as much tilt as I used in the experimental shot.Rob, (from past experience of having to make annual 'portraits' for 75+ scientific staff), might I also suggest that rather than having a 'full frontal' of the subject have your subject face 'about' a 30 to 45 degree angle from the camera lens... but with the face turned directly in towards the lens. you should request that the model NOT lean backwards... but lean 'a little' towards the camera. (elbows usually IN.. but not "tight').
The 'main' light to the 'front of the face and if possible a white 'reflector' close enough to provide some reflected fill from the side almost (but not always) opposite the main light. You might also consider having a lower wattage 'hair-light' (up... sort of higher... and from the rear...) almost 'opposite' from the main light to 'skim' the back of the head (using 'barn doors' to limit the spread'... and not shine right into the lens) thus provide a more 'three-dimensional' image of you subject.
I would also prefer you leave a bit more 'empty space' above your subjects head for a 'portrait'... so... be willing to crop a bit 'tighter' on the lower. You should not need knees unless it is a 'full-body-seated-in-a=chair
Hands, if included, should have the fingers (in most occasions) be slightly 'curled' (very loose 'fist) but there will be occasions (especially with female subjects) where hands can be placed slightly 'overlapping' with palms 'down' on the 'lap'
From what-its-worth-but-your-mileage-might-vary Ken