I know it's been a long time but I wanted to give some post-grout feedback.
Kevin's magic formula worked a treat and has now been in place for over 3 weeks.
I did a little test. I filled the two chips but I didn't tell my wife that I'd done them. The repair is almost invisible. Tonight she finally noticed and said "Did you fix those chips?" which prompted me to do this post.
Thanks Kevin for helping me out free of charge and with a kind heart. I hope I can return the favour some time soon