Context. The transient X-ray source XMMU J004215.8+411924 within M 31 was found to be in outburst again in the 2010 May 27 Chandra observation. We present results from our four Chandra and seven Swift observations that covered this outburst. Aims: X-ray transient behaviour is generally caused by one of two things: mass accretion from a high mass companion during some restricted phase range in the orbital cycle, or disc instability in a low mass system. We aim to exploit Einstein, HST, Chandra and Swift observations to determine the nature of XMMU J004215.8+411924. Methods: We model the 2010 May spectrum, and use the results to convert from intensity to counts in the fainter Chandra observations, as well as the Swift observations; these data are used to create a lightcurve. We also estimate the flux in the 1979 January 13 Einstein observation. Additionally, we search for an optical counterpart in HST data. Results: Our best X-ray positions from the 2006 and 2010 outbursts are 0.3" apart, and 1.6" from the Einstein source; these outbursts are likely to come from the same star system. We see no evidence for an optical counterpart with mB ⪉ 25.5; this new limit is 3.5 mag fainter than the existing one. Furthermore, we see no V band counterpart with mV ⪉ 26. The local absorption is ~7 times higher than the Galactic line-of-sight, and provides ~2 mag of extinction in the V band. Hence MV ⪆ -0.5. Fits to the X-ray emission spectrum suggest a black hole primary. Conclusions: We find that XMMU J004215.8+411924 is most likely to be a transient LMXB, rather than a HMXB as originaly proposed. The nature of the primary is unclear, although we argue that a black hole is likely.