I have the good fortune of having three PhD theses on my desk at present, two where I am the external examiner and one where I am the internal. They are all on interesting topics, one very related to my own work and the other two a little more obliquely related. While reading theses for examination is a chore – you need to make lots of notes and make sure you have sensible questions to probe the student on, it is also a really good way to concentrate on a specific subject and hopefully learn something along the way.
Having picked up the first one to read my heart sank a little as it became clear that there were going to be lots of rather strangled grammar. If you are an aspiring or current PhD student, you need to know that the quality of your science is what will take you through in the end, but your viva is likely to be a hell of a lot easier if the examiner does not need to stop every paragraph to note how you have deviated from the norms of good English.
My strong advice: throughout your PhD, write as much as you can and get feedback on your work. This can be from your supervisor, lab mates or friends or through courses that I imagine all Universities (certainly in the UK) offer on academic writing. Or maybe by writing a blog. Also try, and this can be hard, to thoroughly proofread your final thesis, and get others to do so too. This will make your examiner’s job so much easier, and so much more enjoyable, allowing them to focus on what you have done. Words your means of delivering your message. Make sure they are your friends.