Thank you all so much for your kind comments. I'm afraid if you went too close to the stitches with a ruler, you would find they are not all as even as they might look.
It can be quite quick to do - if you don't keep undoing your work, which I confess I had to do on the smaller areas, to make the stitches finish in the right place when coming to a seam. I also found it better to start and finish on the back and not the front, as I was afraid of making holes in the front with the thicker cotton.
I do like the look the bigger contrasting stitches give it though, thanks for looking.