Ian, what do you currently use?
I cant seem to get a grips with the short wands aparantly designed for stair treads. I feel its too clumbsy and clunky.
At the moment i'm using a prochem "easy grip" stair/upholstery tool and i feel it does a great job, getting in to all the nooks and grannies as well as the curve of the step. Any hand tool with an internal jet will also do a good job.