Once the floor is laid, you give the stones a coat of 50/50 water and pva glue. This makes it easier to wipe off the excess paste when you start mortaring. I discovered that it also gives the stone a nice sheen like a floor polished by decades of mopping and scrubbing.
Wall gaps in the room corners were filled in with more grout and a piece of balsa laid in the doorway to provide framework for the mortar and simulate a threshold. It's starting to come together as a room. Can't wait to get some fixtures and fittings in there.