  We consider actions of completely metrisable groups on simplicial trees in
the context of the Bass--Serre theory. Our main result characterises continuity
of the amplitude function corresponding to a given action. Under fairly mild
conditions on a completely metrisable group $G$, namely, that the set of
elements generating a non-discrete or finite subgroup is somewhere dense, we
show that in any decomposition as a free product with amalgamation,
$G=A\ast_CB$, the amalgamated groups $A$, $B$ and $C$ are open in $G$.
