Aims. We investigate the nature of the massive star [OMN2000] LS1 a nd use these results to constrain the history of star formati on within the host complex W51. Methods. We utilised a combination of near-IR spectroscopy and non-L TE model atmosphere analysis to derive the physical propert ies of [OMN2000] LS1, and a combination of theoretical evolutio nary calculations and Monte Carlo simulations to apply limi ts on the star formation history of W51. Results. We find the spectrum of [OMN2000] LS1 to be consistent with tha t of a P Cygni supergiant. With a temperature in the range of 13.2-13.7kK and log( L∗/L⊙)≤5.75, it is significantly cooler, less luminous, and less mas sive than proposed by previous authors. The presence of such a star within W51 shows that star formati on has been underway for at least 3Myr, while the formation of massive O stars is still on going. The lack of a population of evolved rsupergiants within the complex shows that the rate of form ation of young massive clusters at ages ≥9Myr was lower than currently observed. We find no evidence of internally triggered, sequential star formation within W51, and favour the suggestion that st r formation has proceeded at multiple indepedent sites wit hin the GMC. Along with other examples, such as the G305 and Carina star-f orming regions, we suggest that W51 is a Galactic analogue of the ubiquitous star cluster complexes seen in external galaxie s such as M51 and NGC2403.