On Mon, 28 Jan 2019 18:50:14 -0000, Andrew Mawson wrote:

Has anyone ever used a silicone bead for this? or anything else?

Silicone won't take paint, ridgid fillers will fail. What do you mean
by "caulk" flexible decorators caulk or something else?

Tell him that if he turns his heating off to minimise thermal movement,

With timber humidty causes much greater movement than temperature.
Seems that this is a old staircase and may just be moving in use due
to it's age and wear.

--
Cheers
Dave.