Hiya Horsey. I didn't look at your original thread as I'm afraid I've no experience of steam cleaners other than the commercial ones for cleaning trucks etc.
Reading through the thread, I have to agree with the others. The tiles could do with a really good scrub, whether by hand, or maybe a hired, commercial floor cleaner.
I can't think why you were advised to leave them unsealed. I guess he was being a tad purist. In the real world, it's just not practicable.
Terra Cotta and quarry tiles were traditionally sealed with linseed oil. Worked fine, but rather laborious. As with most things, modern chemical sealers do the job wonderfully. Porous terra cotta, and especially Travetine tiles, for instance, get dirty very easily.
Have a look through this lot for a start. See if you can find your type of tile.............................
http://www.stonetrader.co.uk/search?orderby=position&orderway=desc&search_query=sealer&submit_search=Search