My gut feeling is that this is original, and not altered. There is a similar piece on page 455 of Levines Guide to Knives and their values, 4th edition. The blade shape is different, but it's still a dirk or dagger. It's made by Joseph Allen & Sons, Sheffield.
I read on British Blades forum that the Jonathan Crookes name was bought and was continued to be used.
http://www.britishblades.com/forums/...d.php?p=530511
Look at the marking on the blade. If you could determine the technology used that makes that "dot" pattern, it might date the blade. I'm thinking it's not from the original Crookes. Here's an old Crookes advertisment.
The blade doesn't have the "feel" of a Fairbairn-Sykes to me. I think you've got a really nice original factory made piece.
But I'm certainly no expert.
I'm googling as I'm writing this and found an example of a modern Crookes commando knife.
http://www.fightingknives.info/Home/...6/Default.aspx
Steve
Edit: Here's a good pic of the early logo from a completed Ebay auction.