I've restored and re-fitted some old oak skirting and need to fill some
small gaps along the top. I could use regular caulk and paint the wall
colour over it but would prefer to use a caulk or filler and stain it to
match the Regency oak colour of the skirting. The oak is stable so, in
the absence of a stainable caulk, I'm tempted to use one of the dark
brown Ronseal wood fillers (supposedly stainable) but would prefer
something flexible. Any suggestions?

(Osmo have a caulk that is laughingly called oak colour - they must have
some strange oak trees wherever Osmo is made!)