If you want to get the extra tax relief at the end of each tax year, you’ll need to fill in a self-assessment form or write to your local tax office to tell them.
On the form, you should give the gross personal contributions made (that’s your net personal contributions plus the 20% tax relief).
For example - net personal contributions £100 x 100/80 (basic rate of tax) = gross personal contribution of £125
You shouldn’t include employer contributions on the form but you should include any contributions made by a third party (other than your employer).
You’ll get this relief in the form of a tax rebate, a reduction in your tax liability, or an alteration to your future tax code.