Does understanding the Visuddhimagga require a knowledge of the Pali Canon? I quite fancy reading the Visuddhimagga at some point. I have heard good things about it. However I'm aware that it is a commentary on the Pali Canon and that has put me off starting it. 

How much of the Pali Canon would one need to read before you could profitably read the  Visuddhimagga. All of it (daunting), none of it or some. If it is some then which some. What are the parts of the Pali Canon that it would be best to read before the Visuddhimagga?
Like much of Buddhism, your understanding of the text is probably relative to not only your familiarity with Buddhist concepts and theory, but also your own cultivation of the practices themselves.

We went through some of the Visuddhimagga in Winnipeg last year, and I had to explain and elaborate on a lot of the technical aspects of the book using the canon and commentaries. But much of it is easily accessible to the neophyte, especially the first two sections.

Absolutely, reading the entire tipitaka would be the best way to prepare for the Visuddhimagga; go for it! If you can't handle the dauntingness of such a task, beware, because a partial read of the dhamma can give you a lopsided understanding. That's what texts like the Visuddhimagga are for, to give you an overview.

A good way to get a fairly solid overview is to read a single collection, like the Digha or the Majjhima nikaya. Read both if you can, but read them in their entirety, so you get a broad look at the types of things the Buddha taught. The MN can be read one sutta at a time - you can make a chart and check off every time you finish one sutta (there's 152 of them). Bodhi's translations are good. Check out https://play.google.com/store/apps/details?id=org.yuttadhammo.buddhisttexts&amp;hl=en

Practice meditation too; samatha if you want to understand the samadhi niddesa aspect, vipassanā if you want to understand the paññā niddesa. In fact, don't read the tipitaka unless you're meditating as well; I would imagine it difficult to understand (or easy to misunderstand) without actually practicing.

But in the end, what's to worry about? It's all good stuff, tried, tested, and trusted by meditators around the world for thousands of years. Good luck :)