  We develop a method to identify the spectroscopic signature of unresolved
L-dwarf ultracool companions, which compares the spectra of candidates and
their associated control stars using spectral ratio differences and residual
spectra. We present SpeX prism-mode spectra (0.7-2.5 micron) for a pilot sample
of 111 mid M dwarfs, including 28 that were previously identified as candidates
for unresolved ultracool companionship (a sub-sample from Cook et al. 2016;
paper 1) and 83 single M dwarfs that were optically colour-similar to these
candidates (which we use as `control stars'). We identify four candidates with
evidence for near-infrared excess. One of these (WISE J100202.50+074136.3)
shows strong evidence for an unresolved L dwarf companion in both its spectral
ratio difference and its residual spectra, two most likely have a different
source for the near-infrared excess, and the other may be due to spectral
noise. We also establish expectations for a null result (i.e. by searching for
companionship signatures around the M dwarf control stars), as well as
determining the expected outcome for ubiquitous companionship (as a means of
comparison with our actual results), using artificially generated unresolved
M+L dwarf spectra. The results of these analyses are compared to those for the
candidate sample, and reasonable consistency is found. With a full follow-up
programme of our candidates sample from Cook et al., we might expect to confirm
up to 40 such companions in the future, adding extensively to the known desert
population of M3-M5 dwarfs.
