  New NTT/SOFI imaging and spectroscopy of the Wolf-Rayet population in
Westerlund 1 are presented. Narrow-band near-IR imaging together with follow up
spectroscopy reveals four new WR stars, of which three were independently
identified recently by Groh et al., bringing the confirmed WR content to 24 (23
excluding source S) [..] A quantitative near-IR spectral classification scheme
for WR stars is presented and applied to members of Westerlund 1. Late subtypes
are dominant, with no subtypes earlier than WN5 or WC8 for the nitrogen and
carbon sequences, respectively. A qualitative inspection of the WN stars
suggests that most (75%) are highly H-deficient. The WR binary fraction is high
(>62%), on the basis of dust emission from WC stars, in addition to a
significant WN binary fraction from hard X-ray detections according to Clark et
al. We exploit the large WN population of Westerlund 1 to reassess its distance
(~5.0kpc) and extinction (A_Ks ~ 0.96 mag), such that it is located at the edge
of the Galactic bar, [..]. The observed ratio of WR stars to red and yellow
hypergiants, N(WR)/N(RSG+YHG)~3, favours an age of 4.5-5.0 Myr, with individual
WR stars descended from progenitors of initial mass ~ 40-55 Msun. Qualitative
estimates of current masses for non-dusty, H-free WR stars are presented,
revealing 10-18 Msun, such that ~75% of the initial stellar mass has been
removed via stellar winds or close binary evolution. We present a revision to
the cluster turn-off mass for other Milky Way clusters in which WR stars are
known, based upon the latest temperature calibration for OB stars. Finally,
comparisons between the observed WR population and subtype distribution in
Westerlund 1 and instantaneous burst evolutionary synthesis models are
presented.
