The affected toilet must be on its own run initially, before connecting with the other pipes, blockage would be in this initial run if no other areas are affected. Classic symptoms are there, the water is slowly seeping away over time, so its going somewhere! I'd be wary of chucking too much water down in a short space of time, should the problem be affecting the whole house, you dont want it reappearing (somewhat dirtier) from the downstairs W.C.
Without being on site it is difficult to advise where else to look, any possibility of a sketch showing the footprint of the building and
all toilets, gullies etc, the 2 rodding eyes and any manholes within a 100m radius?