need a flexible filler

I like silicone caulk; you can also use "siliconized" latex caulk. In the US these are available in a number of colors. I have mixed the available colors to achieve the color match needed. It is also possible to start with clear silicone caulk and add paint (I have used automotive enamel). Maybe even the colored powders used to alter mortar (cement) coloration would work. You would have to experiment.

Jim

thanks Jim. Is this anything like the silicon sealants (commonly clear or white here in the UK) that I would use round shower-trays or the bath? Or is it a different thing altogether. I need something pretty hard since this will be showing in anything up to 3/4" gaps and will need to take a pounding from heavy boots.

Yes, it is silicone sealant (RTV). A 3/4" gap may be too much combined with the heavy traffic. Try a small experiment.

Another idea is to locate a supplier of rubber stocks. Look for a material in ~3/4" thickness which could be cut into strips of the correct dimension to slip in the gap. Silicone sealant could be used on both sides of the rubber strip to adhere it to the base materials.

Jim

yes, that could work. These gaps are extremely variable but now you mention it I suppose I could also use oak strips (its old oak which has worn and splintered unevenly) and use the sealant as a small gap filler. I'll try this.

Many thanks.

I bought a floor leveler at Lowe's USA that is flexible, and used it to fill some gaps in the kitchen floor of my 100+ year old house where the "give " of the floor caused the tile to crack. Then I laid the new floor (vinyl tiles). It works well and is definitely flexible, as opposed to portland cement levelers.