Do you have any proof that you originally sent the forms?
What was the reason the original backdating?
You'd make a new claim online, and then use the past dates when asked when you want to claim for. It will take an absolute age because it'll probably be rejected initially, and then referred to a decision maker. In all honesty, a very, very small proportion of retrospective claims are granted. The last time I helped with one of these (beginning of 2015), you needed proof that the original forms were returned on time and received, and proof that you chased up their receipt using more than one method (email, call, physical visit) on more than one occasion.
It's worth putting the claim in, but unless you've got that, I wouldn't rely on receiving the money.