Authorised unpaid leave
If you aren't being paid, then you aren't paying pension contributions, so this will reduce your benefits. But if you like, you can catch up on the lost benefits by paying additional pension contributions (APCs) and your employer will share the cost with you, by paying 2/3 of the cost.
But you only have 30 days from returning to work to take up this option. If you choose to pay APCs outside this deadline, you will have to pay the full cost.
You won’t be paid during any periods of unauthorised leave, so naturally you won’t be paying any pension contributions. In this case there is no option to catch up on the lost benefits by paying APCs.