The trouble with all of the above is that the material you are using to fill the gaps, as well as not matching the timer floor, will expand and contract at a different rate to the boards leaving you with smaller but more numerous cracks. I'm afraid there is no trouble free way to do it, the correct way is as follows. Any gaps over 1.5mm wide should be filled using wedge shaped strips of the same timber forced down into the gap with a mallet and stuck using PVA adhesive. when you have sanded off the whole floor save the sanding dust from the final sanding (finest paper) mix this dust with a spirit-based (not water-based) filller (Lecol or Legante) and "grout" all of the floor. A final very fine sanding then apply your seal. The easiest to use are waterbased polyurethane seals (Bona). Do the work in the winter when your heating is on and the gaps are at there largest. You will find that a lot of the draughts are comming from around the perimeter, under the skirting, if the gap under the skirting is small you could fill it with silicon sealant. Good luck.