  Context. The Galactic chemical evolution of sulphur is still under debate. At
low metallicities some studies find no correlation between [S/Fe] and [Fe/H],
others find [S/Fe] increasing towards lower metallicities, and still others
find a combination of the two. Each scenario has different implications for the
Galactic chemical evolution of sulphur.
  Aims. To contribute to the discussion on the Galactic chemical evolution of
sulphur by deriving sulphur abundances from non-LTE insensitive spectral
diagnostics in Disk and Halo stars with homogeneously determined stellar
parameters.
  Methods. We derive Teff from photometric colours, logg from stellar
isochrones and Bayesian estimation, and [Fe/H] and [S/Fe] from spectrum
synthesis. We derive [S/Fe] from the [S i] 1082 nm line in 39 mostly cool and
metal-poor giants, using 1D LTE MARCS model atmospheres to model our
high-resolution NIR spectra obtained with the VLT, NOT and Gemini South
telescopes.
  Results. We derive homogeneous stellar parameters for 29 stars. Our results
argue for a chemical evolution of sulphur that is typical for alpha-elements,
contrary to some previous studies. Our abundances are systematically higher by
about 0.1 dex in comparison to other studies that arrived at similar
conclusions using other sulphur diagnostics.
  Conclusions. We find the [S i] line to be a valuable diagnostic of sulphur
abundances in cool giants down to [Fe/H] ~ -2.3. We argue that a homogeneous
determination of stellar parameters is necessary, since the derived abundances
are sensitive to them. Our results ([S/Fe]) show reasonable agreement with
predictions of contemporary models of Galactic chemical evolution. In these
models sulphur is predominantly created in massive stars by oxygen burning, and
ejected in the ISM during Type II SNe explosions. Systematic differences with
previous studies likely fall within modelling uncertainties.
