If immigration gave you the form to get a witness to sign it would normally mean the witness isn't required to attend. They would normally tell you or your wife. It may have changed but when we did our extension in August that form wasn't available for download. We just had our witness sign and provide a copy of their ID and house book.
What's your travel date? That visa is valid for six months from the date of issue. So if your travel date is a way off they might just be holding off issuing.