Witness statement should not be put in the bundle of documents.

There is a theoretical justification for this, but the main reason is convenience.¹

Witnesses will often be referred to documents in the bundle and when making submissions about witness evidence you will often move from the statement to the documents it talks about.

If the documents and statements are in the same bundle this involves a lot of flipping back and forth. Much easier to be able to keep the statement open in front of you while thumbing through the documents.

1. The theoretical justification is that witness statements are a form of examination in chief, not documentary evidence.