It's more a visual thing, hence the link to the picture.
Even running the basic "SFML Works!!!" example shows that the circle is not in the bounds of the RenderWindow. Given the dimensions of the window and the circle, it should all fit.
Anyway, having further looked at the problem, it appears to occur on my laptop, but not my desktop, so it looks like it is not SFML's fault at all.
The question then becomes, what on earth is causing it.