filling a gap

[Graham & Brenda]

In our lounge there is a small gap between the edge of the oak floor and the heavy flags over the solid part of the floor. Because the oaks moves (albeit not visibly so) the cement based infill has crumbled and the draught blows through from the ventilated cave below. The gap is between 1 and 4mm. It is not very accessible from below but I've put expanding foam where I can get to it.

Any ideas please for a flexible gap filler that won't be unsightly in the join across the lounge floor.

Thanks,

Graham

[Nick Trollope]

    Sikaflex - used for caulking boats?

Any other Mastic would do, I'd imagine..