Help - can I split listing ?

Not from the same listing you can't. As long as there are no bids, best way would be to edit so the item you are selling separately is no longer in it, then create a new listing for that item and point the buyer to that new listing. You could do this several times if need be for different parts.

I wouldn't.

If you have already created the listing, then stick with it. If you split it up after the listing has gone "live" you may / will find that you are going to annoy a potential customer who is interested in the whole set.

When we get this requests we answer with something similar to this "Hi there, no sorry we wont be splitting this set up ourselves. But if you did win this auction, you are welcome to relist the parts that you don't want and sell them yourself - who knows you may even make a profit!"

This way, you are providing them with a solution, without disrupting any other potential bidders.