It is not a backlog if the ICO has not noticed how many requests are outstanding and only becomes a backlog when the ICO starts to get involved.
A 2.5% compliance rate (at times) and 650+ backlog should never have been acceptable and senior management should have been fighting tooth and nail to overcome the problems, even if there was a vacancy freeze.
I have a feeling that the ICO will be reinventing themselves post election to justify their role and even if they don’t have the budget for a lot of things, will be pushing interpretation of the law and holding organisation to account that way.