Filler / sawdust should be ok but it will slump as it dries. You could try to put some 2 pack filler into the bottom of the holes and bring it up to 1 - 2mm below the level of the floor and this will guarantee that you don't get any slump.
@Mickie_Cricket started
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Filler / sawdust should be ok but it will slump as it dries. You could try to put some 2 pack filler into the bottom of the holes and bring it up to 1 - 2mm below the level of the floor and this will guarantee that you don't get any slump.