Restoring quarry tiles

[Jewkesy]

Hello all,

This is my first post on here so please bear with me if I do or say anything stupid!

I recently bought a house (built in 1930s, ex-Council) that I am currently working on renovating. When I took up the carpet in the hall I found that the original quarry tile floor was still in place beneath the layers of carpet and underlay. Naturally I want to restore the tiles and keep the floor rather than replace it with something with less character. Work on the house is coming towards the end now so last night I set about cleaning the tiles up as best as possible using LTP Grimex (as recommended elsewhere on this board).

I left the tiles to dry out overnight and this is how they now look...

http://flic.kr/s/aHsjwSSHSU

Whilst the treatment has removed most of the dirt, a few particularly stubborn black marks are still visible (see photos) and generally the tiles look more dull than previously.

Has anyone got any suggestions firstly as to what the black marks might be and how I can remove them, and secondly how I can now treat ther tiles to restore some of their colour and make them look more tidy?

Many thanks for any suggestions. I really love the tiles so want to rescue them if at all possible!!

Jewkesy