Spectral age modelling of the `Sausage' cluster radio relic
  CIZA J2242.8+5301 is a post-core passage, binary merging cluster that hosts a
large, thin, arc-like radio relic, nicknamed the `Sausage', tracing a
relatively strong shock front. We perform spatially-resolved spectral fitting
to the available radio data for this radio relic, using a variety of spectral
ageing models, with the aim of finding a consistent set of parameters for the
shock and radio plasma. We determine an injection index of
$0.77^{+0.03}_{-0.02}$ for the relic plasma, significantly steeper than was
found before. Standard particle acceleration at the shock front implies a Mach
number $M=2.90^{+0.10}_{-0.13}$, which now matches X-ray measurements. The
shock advance speed is $v_\mathrm{shock}\approx2500$ km s$^{-1}$, which places
the core passage of the two subclusters $0.6-0.8$ Gyr ago. We find a systematic
spectral age increase from $0$ at the northern side of the relic up to $\sim60$
Myr at $\sim145$ kpc into the downstream area, assuming a $0.6$ nT magnetic
field. Under the assumption of freely-ageing electrons after acceleration by
the `Sausage' shock, the spectral ages are hard to reconcile with the shock
speed derived from X-ray and radio observations. Re-acceleration or unusually
efficient transport of particle in the downstream area and line-of-sight mixing
could help explain the systematically low spectral ages.
