What I'm looking at in the background texture is that where there's a horizontal row of pixels missing from the text, there's also one missing from the repeating background texture, which you can see if you look closely. What that suggests to me is that your back buffer is not the same size as your window client area.
You can easily verify if I'm correct by running it in full screen mode instead of windowed.
For anyone else:
PresentParams.BackBufferHeight = form.ClientRectangle.Height;
PresentParams.BackBufferWidth = form.ClientRectangle.Width;
PresentParams.BackBufferHeight = form.Height;
PresentParams.BackBufferWidth = form.Width;
Silly mistake.. thanks !