  We present photometry and time-series spectroscopy of the nearby type Ia
supernova (SN Ia) SN 2015F over $-16$ days to $+80$ days relative to maximum
light, obtained as part of the Public ESO Spectroscopic Survey of Transient
Objects (PESSTO). SN 2015F is a slightly sub-luminous SN Ia with a decline rate
of $\Delta m15(B)=1.35 \pm 0.03$ mag, placing it in the region between normal
and SN 1991bg-like events. Our densely-sampled photometric data place tight
constraints on the epoch of first light and form of the early-time light curve.
The spectra exhibit photospheric C II $\lambda 6580$ absorption until $-4$
days, and high-velocity Ca II is particularly strong at $<-10$ days at
expansion velocities of $\simeq$23000\kms. At early times, our spectral
modelling with syn++ shows strong evidence for iron-peak elements (Fe II, Cr
II, Ti II, and V II) expanding at velocities $>14000$ km s$^{-1}$, suggesting
mixing in the outermost layers of the SN ejecta. Although unusual in SN Ia
spectra, including V II in the modelling significantly improves the spectral
fits. Intriguingly, we detect an absorption feature at $\sim$6800 \AA\ that
persists until maximum light. Our favoured explanation for this line is
photospheric Al II, which has never been claimed before in SNe Ia, although
detached high-velocity C II material could also be responsible. In both cases
the absorbing material seems to be confined to a relatively narrow region in
velocity space. The nucleosynthesis of detectable amounts of Al II would argue
against a low-metallicity white dwarf progenitor. We also show that this 6800
\AA\ feature is weakly present in other normal SN Ia events, and common in the
SN 1991bg-like sub-class.
