We present photometry and time series spectroscopy of the nearby Type Ia supernova (SN Ia) SN 2015F over ?16 d to +80 d relative to maximum light, obtained as part of the Public ESO Spectroscopic Survey of Transient Objects. SN 2015F is a slightly sub-luminous SN Ia with a decline rate of ?m15(B) = 1.35 ± 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 ?6580 absorption until ?4 days, and high-velocity Ca?ii is particularly strong at 10 d at expansion velocities of ?23 000?km?s?1. 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 >14?000?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 ?6800 Å 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 Å feature is weakly present in other normal SN Ia events and common in the SN 1991bg-like sub-class.