Thanks for the help and sensible amounts of skepticism everyone. =).
@Appo: The theory is good. I may just change the yspeed condition to check whether ground = 1 as I do have the same grass effect on slopes, which would mean that y speed <> 0. Thanks for the idea ^^. And don't worry, I've had threads filled with much worse.
@zenor: Would I be right in saying that because the sensors don't change layer then there won't be any of the clichéd 'falling thorugh the floor' issues? That's something I'd never thought of before. I may go with Appos idea due to the (apparent) ease of coding. thanks for the suggestion though, I'll keep it in mind.
@Rael: Ah, that makes sense. I'd been looking at collisions instead of overlapping. Because you can't negate the collision condition I was having problems. I'll give that a shot as soon as I can.
@DimensionWarped: Well, peculiar I take as a compliment. I guess I'll just have to live with the cheesy part which is a shame, because I hate cheese (the foodstuff, not the chao)