Try downloading Steve's Lomdon.ZIP from the Lemming Level List Game. It contains his recreation of 'The Bridge is Breaking Down', and, for some reason, a larger bricklemmus style file which this level requires.
edit: And this 'Brand New Lemming Harvester' level is annoying too!
edit edit: Finally passed it! It's a clever level, and the payoff is good too, the timing is just so tight though! Gunpowder, did you by any chance test it and fast forward it as the last lemming walked in? Because there's a bug that lets lemmings walk in at the speeded rate while the level fades at the standard rate, hence giving the player a second or two longer. Brand New Lemming Harvester is probably possible without doing this, but it's extremely tight either way. Unless there's a quicker route I'm not seeing . . . What's the shortest time you've completed it in?