In June I bought a pair of trousers in size 14. I liked them a lot, but I now have to wear a belt to keep them up, so I went back to try a smaller pair of the same style. (I've just sent most of my size 18 wardrobe to the charity shop, so I have some catching up to do!) I tried on the 12 and they fit perfectly, but the colour I want was no longer in stock. I tried on the right colour in 10 and they did up, but they were horribly tight around my rear and thighs. Definitely too small.
Reader, I bought them.
M&S accept returns up to 35 days later. So the size 10 trousers are resting in my wardrobe with their receipt. 5 weeks of 5:2, let's see what happens. I'll post back in early August and let you know!