  We investigate the variation in the MMHT2014 PDFs when we allow the heavy
quark masses $m_c$ and $m_b$ to vary away from their default values. We make
PDF sets available in steps of $\Delta m_c =0.05~{\rm GeV}$ and $\Delta m_b
=0.25~{\rm GeV}$, and present the variation in the PDFs and in the predictions.
We examine the comparison to the HERA data on charm and beauty structure
functions and note that in each case the heavy quark data, and the inclusive
data, have a slight preference for lower masses than our default values. We
provide PDF sets with 3 and 4 active quark flavours, as well as the standard
value of 5 flavours. We use the pole mass definition of the quark masses, as in
the default MMHT2014 analysis, but briefly comment on the $\overline{\rm MS}$
definition.
