Following the crit, my tutor suggested that for the desk scene where it is supposed to be dark, I use a dark blue tinted light. I've been experimenting with these results:
I do think it more effectively gives the impression of night-time, whilst still ensuring everything is visible, so I will be using this style of light in my animation. However I still can't seem to get the correct shadows, even though they are enabled they look barely visible on the background props. I increased the light radius and number of shadow rays as I wanted them to look softer and more realistic, and I do like how this has been achieved for the lamp, but clearly I'm not doing something right. Perhaps I haven't set up enough lights. But, time is of the essence now so I am going to focus on the animation.
I've also added cardboard textures to the background props. I'd have liked to have painted some rough cloud/tree/castle designs onto the cardboard, so it looks as if he had created and painted them himself, but I found it quite difficult to distinguish what went where on the UV maps. I'll definitely have to practice this in my own time so I can use my own textures in future animations.