I've not worked out a way to flush it out but had tissue in there to take it off until the gears were dry and then re-oiled.
I've yet to strip the chassis down again to spray, so if there is a better method I shall give it a go. Definitely made a difference to the running though, as it was making some horrible noises previously.
I was thinking more just the ABC gearbox flushing, but may give it a go for the chassis, although it'll be tricky in a 4th floor flat with little man running around.