the routemaster sound was a real 303 put through a boss distortion pedal
( the yellow one )


the suf sound was a 303 straight into a channel of a mackie 24/8/2 with the gain turned up full and the mids and highs up , then just keep the fader low
and you'll need a noise gate.
you can get almost the same sound from smaller mackie desks too
very smooth , especially with a bit of chorus on it too.
great for melodic lead lines

have you tried NI guitar rig?
thats pretty good for software distortion.