law/guidance around backdated payments with regards to "PIP review delays"

I was wondering what is the law/case notes around backdated payments due to delayed reviews?

Your news article says that its a lottery, which i can believe, but what is the actual position of the law on this point?

Should payments be from the date of the submitted review form or from the date of the decision?

Its something worth knowing so that we can push the DWP for money that we might be owed if they choose the "wrong" date.

Thanks