  Line intensity mapping opens up a new and exciting window for probing
cosmology and fundamental physics during the Epoch of Reionisation, extending
to redshifts previously untested by galaxy surveys. The power spectra of these
line fluctuations are a promising tool to test gravity over a large range of
scales and redshifts. We simulate cosmological volumes of 21cm fluctuations in
general parametrisations of modified gravity, in order to calculate the
corresponding power spectra, where additional parameters are the initial
condition of matter perturbations $\alpha$ and the scale-dependent modified
gravity parameter $Y$ (also known as $G_\mathrm{eff}$) that measures deviations
from GR in the Poisson equation. We show the impact of these model-independent
modifications of gravity, to either delay or expedite reionisation. For the
21cm intensity mapping survey to be performed by the SKA mission, we forecast
the ability of line intensity mapping to constrain the parameters $Y$ and
$\alpha$ at redshifts $z=6-11$, where $Y$ is assumed constant during this epoch
(but without requiring constancy at all times). In our most conservative
scenario, the $Y$ parameter can be constrained at the tens of percent level,
while for improved modelling of foregrounds as well as of the (mildly)
non-linear regime, up to sub-percent level constraints are attainable. We show
the impact of jointly estimating reionisation model parameters and
corresponding parameter correlations, as well as of foreground removal. We
note, that tomography is crucial to break degeneracies and for constraints not
to degrade significantly when adding reionisation model parameters, with most
constraining power coming from the redshift bins $z=7-10$ where the shape of
the 21cm power spectrum is evolving fastest.
