EDIT: People have commented saying my post title is misleading, so adding clarification here. I got one minor driving fault on my test, and one serious fault, which is obviously why I failed. My minor fault was for hesitation at the beginning of my test, and my serious fault is described below. It was marked as a serious 'use of mirrors - change direction' as I was moving into the spiral part of my lane on the roundabout and didn't make it clear enough that I was checking my mirror. Definitely will learn from this in future, and I'm not trying to pretend that I am the world's best driver. I had wrongly assumed that having 'failed' in my title, adding the 'failed' flair and explaining in my post what had resulted in my fail was clear enough, but I posted this not long after getting back and maybe should have made this even more clear.

Original Post: Just got back from my test in Doncaster. Had a really good drive through rush hour traffic, a good parallel park and controlled emergency stop. Going over the College Roundabout (3 lane spiral) and someone undertook me and had come flying past my left side as I was following the spiral bit round, did mirror checks and was in my lane but failed due to 'not checking my mirrors.' I saw the examiner out of the corner of my eye move as if he was going to take the wheel but didn't. Just wondered what to do if this situation arises in the future? My instructor said this sort of thing happens frequently on that roundabout.

Pretty deflated but proud for giving it my best shot and only getting one minor!