If the tiles are on concrete or stone then your problem may keep coming back if the substrate (i.e. concrete or stone) is also cracked below. If they are on wood, then part of your problem may be movement and again the problem may keep returning. If neither of the above (perhaps even in any case) you may wish to consider replacing the cracked tiles provided you know the source and are able to get a match. A permanent solution is better than a short term crack-filling one because of the rigidity of the tiles (even thermal movement due to changing temperatures will re-open the cracks). The cracks will be "hairline" type and are likely to be nigh impossible to fill. Check with a company called Conren, tel. 01978 661991, to see if they have a flexible overpaint product which might suit you if the tile replacement (some or all) option is ruled out.