Sounds like a solid start. First, make every line item mandatory—no blank spots, no freeform notes that can be ignored. Then set up cross‑checks: every expense should have a supplier ID, a cost centre code, and a timestamp. If the system flags a purchase that doesn’t match any known vendor or cost centre, put it on hold until you verify it. Keep a “blacklist” of vendors that’ve slipped through before, and enforce a minimum approval threshold for any amount over a set value. Also, a quick audit trail of who approved what, and when, can expose the ones who slip under the radar. Finally, schedule random spot‑checks; the surprise factor stops people from gaming the system.