I also used to work for a large diy company. Th grout pens are excellent, but you cant beat a steamer.
People often refer to pressure washers, as a steam cleaner. But the steam cleaning mentioned, will in fact be a vapour steam application.
We have used these on many occasions, and cant speak highly enough of their performance. They work by penetrating right into the porous surfaces, that would be missed, via a chemical based approach initially. They really cant be beaten, for the deepness, with which they clean.