Symz, I use a combination of MS Paint and Photoshop. I'm really comfortable using Paint and could draw these diagrams without Photoshop, but the layers and opacity settings in PS make the process a lot quicker.
Haljackey, I don't mind the requests so long as you guys realize it may take a while. I have never seen a render of the Sunlife buildings so I don't know what to draw for their diagrams.
Anyhow, I was getting bored of the London Towers so this afternoon I drew the old TD building. Thought you guys would enjoy a little step by step as well. Sorry about the watermark, just taking Mike's advice.