As you can see from the pictures, my two finished squares are different sizes. The mitred square (green centre) is the correct tension and the corner to corner square (blue centre) is bigger. The difference shows most in the border and edge triangles rather than the centres. This was a bit confusing as I'd used the same needles, same yarn, same stitch but the tension is different. Then I remembered the plaster. I use Hiya Hiya Sharp interchangeable needles which need to be re-named 'very sharp'. I'd been knitting a jumper in the round fairly quickly and had caught the end of my finger with the needle a couple of times so, rather than bleed over my knitting, I put a plaster on my finger. The yarn doesn't slip over a plaster in the same way that it slips over skin so when I swapped back to my blanket square to do the border and triangles, the tension was different. The square is only one stitch awry over 10 cm but on a 20 cm square that becomes two stitches which is visible. This is why tension is important when knitting!
We now have a new rule for tension -- no plasters!