PIP Letter Lost

The only standard letter sent out between those dates will be the annual uprating letters, as Gordon has already said the only letter showing entitlement and points scored for award is the original decision letter.
The statement of entitlement letter states on the front the start date of the award, which is probably what the council lady is looking at, however under the mobility scores on the second side will categorically state your total score for the mobility part of PIP is XX points you've been awarded the xxxx rate from xxx date to xxx date. So long as this is in date currently the council will have to accept this is the only document you can provide to prove points awarded and length of award.
Scan a copy specifying she look at the length of award.
Good luck