Hi Richard
seems general good advice so far
the key is to test an area first .................. get as much information on the floor such as floor type, sealer used before, current maintenance used, damages, etc
the last guy was quite right to say many large and small companies will avoid this type of flooring.
generally most are okay simply to strip and clean using a soft brush and rotary
with a wet pick up, however if linseed has been used to seal, this cannot all be removed as most of the sealer will be deep in the tile because its so porous
it will have soaked up the sealer to some depth, but that is ok
the tile will be cleaner and all you do is top up with the same ie linseed, the same if it is a wax sealer or primer..
many companies now are going back to wax and primers away from the synthetic modern acrylics etc, as the acrylics have caused problems with the tile surface when being removed, and cause damage to the finish.
also the wax covers a multitude of imperfections of this type of cotto and helps to look uniform.
under no circumstances use very aggressive pads or brushes on smooth terracotta unless you want a large insurance claim, the top of the clay tile has a very fine soft layer that once removed will reveal a courser denser mix of clay that looks horrible as a finish, this tends to look like shadowing which looks ok when wet but aweful when dry and you will get a call back, but basically you will have removed the top layer.
So try a small area use a good alkali cleaner allow dwell time and a thorough rinse, if you get a good result check with custy and charge well, you will need de-humidifier for drying and several days or weeks before sealing, with like sealer that has already been used before.
hope this helps,
Karl