Hi Neil
Welcome to the world of mirror making!
I was looking at your photos and if the tool is dry and clean, as Stephen says it is not making full contact with the mirror. It looks like you have ground off the tiles' glazed surface in parts but not everywhere. The tiles should look the same colour all over.
Some people make a cardboard or thin plywood template to monitor the curve. More commonly you use the pencil (or 'sharpie') test to tell if the tool and mirror are properly mated:
http://gotgrit.com/fsharpie1.php
https://stellafane.org/tm/atm/grind/...ml#Pencil-Test scroll down the page to see the description and what to do to bring tool and mirror into contact.
Hope that helps
Rod.