Give it a little time. I've been riding Brooks saddles since the early 1990s (you'd be surprised at just how difficult they were to buy then) and can say that a large part of it is breaking in your butt, not the saddle. After a week or two of daily riding all should be well. The fact that you are slipping around indicates that you haven't ridden the saddle much at all. The slipping will stop with a few more miles.

Don't chop it as it won't change anything except the look of the saddle, don't futz around with the tensioner screw (I've adjusted one of my saddles twice since the early 1990s to give you a clue about how often it needs to be done), and don't soak the thing in any oils or creams - you'll just foul it up. A light coat of Proofhide to help protect it from the elements is all you need.

That being said, some people just don't like Brooks saddles, you might be one of them.