Hi Richard That is a very hard question to answer, firstly you need to look at the condition of the tiles do they need cleaning or do they need stripping and re sealing. If the first then a weak dilution of STS then brushed in with something like a floor mac or light rotary, use a grout brush and that will clean the surface.
On the other hand from experience the client will say cleaning but in reality they mean stripping and re sealing. The issue with that is to find out what they have been sealed with in the first place. Because of the very porous nature of Terracotta one of the biggest problems is the drying time between strip/clean and re seal. If you have never done this before or been on a course then I would suggest you do train first.
If you seal too soon, and depending on the sub floor it can take several days, you will trap moisture and enter a whole new level of pain.
If you post some pictures then I am sure someone will advise on whats needed.