Go to People

Select the team member you'd like to backdate pension contributions for

Go to the Work & Pay tab

Calculate the £ values of how much needs to be contributed

Add the different to the contribution for the current month

In the Pension box, hit Update, then Update the Calculation method from % to £.

Enter the value of the amount

You should see contribution values in the Pension box refresh. These values will instantly update on the Payroll tab.