Replacing a few slates - Roofing job in Manchester, Lancashire

It was cheapest quote and Rodney was early. We went up the roof and had to inform him that he missed a tile but don't think he meant to. Nice enough guy. Said that he adds Thompsons weather seal to the cement but if you read the instructions online, it says that you should spray or roll on so we do wonder whether by adding it to a wet cement it might be damaging to the cement as it repels water and well...wet cement does contain water!