I have read a lot of discussions about filling gaps in floorboards but have not read anything yet except for pushing in bits of died rope that might help with our problem. The standard solution of mixing adhesive with sawdust has been a hopeless failure. We had our Victorian floorboards in 2 rooms and hallway professionally sanded, stained and gaps filled with a mixture of adhesive and the sandings or sawdust. This filler has since popped out of about one third of all the gaps. It's now not an option to start again. To remove the parts of filler which remain in place would be a gigantic job and would scratch the varnish on the floorboards. The gaps are in patches all over the floors, so really I am looking for a filler which I can stain to match with which I can fill these patches and continue to use as and when more of the existing filler comes out in the future. The filler needs to have a lot more stick and grip than the standard used adhesive/sawdust mix and needs to be flexible to prevent it popping out as it has as soon as we began to walk on the floor.