In the TQFT formalism of Moore–Tachikawa for describing Higgs branches of theories of class $${\mathcal {S}}$$
S
, the space associated to the unpunctured sphere in type $${{\mathfrak {g}}}$$
g
is the universal centraliser $${\mathfrak {Z}}_G$$
Z
G
, where $${{\mathfrak {g}}}=Lie(G)$$
g
=
L
i
e
(
G
)
. In more physical terms, this space arises as the Coulomb branch of pure $${\mathcal {N}}=4$$
N
=
4
gauge theory in three dimensions with gauge group $${{\check{G}}}$$
G
ˇ
, the Langlands dual. In the analogous formalism for describing chiral algebras of class $${\mathcal {S}}$$
S
, the vertex algebra associated to the sphere has been dubbed the chiral universal centraliser. In this paper, we construct an open, symplectic embedding from a cover of the Kostant–Toda lattice of type $${{\mathfrak {g}}}$$
g
to the universal centraliser of G—extending a classic result of Kostant. Using this embedding and some observations on the Poisson algebraic structure of $${\mathfrak {Z}}_G$$
Z
G
, we propose a free field realisation of the chiral universal centraliser for any simple group G. We exploit this realisation to develop free field realisations of chiral algebras of class $${\mathcal {S}}$$
S
of type $${{\mathfrak {a}}}_1$$
a
1
for theories of genus zero with $$n=1,\ldots ,6$$
n
=
1
,
…
,
6
punctures. These realisations make generalised S-duality completely manifest, and the generalisation to $$n\geqslant 7$$
n
⩾
7
punctures is conceptually clear, though technically burdensome.