You can hire a Lawyer if that will make you feel better but my advice below:
-Don't panic else you will mess things up even more (I know it's not easy but doable)
-If possible, take a picture of the front of your office and attach, if you have office logo on your building even better. ( sounds stupid but anything to convince this people and clear your name)
-Attach T4, paystubs and everything you have attached before again ( does no harm plus it probably didn't go with your documents earlier because of a possible system glitch)
-If you still work there, get another updated letter from your employer to confirm your employment details with a date after the fairness letter was written.
-Just keep thinking of anything you feel might make your case strong and add on ( nothing is too much to add at this time, if you think it, add it, remember you have been accused, so you need to clear yourself)
- Remember you have 15 days and you will waste time trying to get a good lawyer and responding in time and very well.
Best of luck.