Hi again everyone, thanks so much for all of your advice and support. I told him about the grout this morning and even though he was insistent that the adhesive was was grout as well, I stuck my ground and told him that I wanted it changed. He said he'll do it, so fingers crossed he'll do a good job of it.
Faithhealer, I'm just hoping it is all okay when he's finished as I can't afford to get it up and start over. There's no way of recouping any money back from him for the work done, nor the cost of replacing tiles and UFH, so whatever the end result, I'm stuck with it. 🙁