While people are already thinking about filling gaps in floors (see an
earlier post about cork beading) I have a similar question.

We have, courtesy of the previous owners, sanded and varnished
floorboards in our living room and hall. The gaps between boards range
from basically nothing to about 4 or 5 mm. My SO doesn't mind them but
they bug me because I can feel each board under foot. I would like to
fill the gaps with something but, as it's a big job that think could
ruin an otherwise very good floor, I would like your advice.

Things I have read about:

Cork: Ok but will it last and how do I get it in the smaller gaps.

Sealant (silicone or otherwise): If flush I don't think it will last.

Caulking (as done on ships): Not bad but hard work and will likely leave
ridges.

Slivers of wood: Nice idea if you can get hold of them. Will need to
re-sand and varnish floor though.

Any other ideas?

If you think that I am being bugged by nothing feel free to say.
Everyone else I have spoken to (except what I am sure is B&Qs oldest
employee) things that I am mad for even caring.

Cheers

Graham