Hello, Morten, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
The usual treatment of pension contributions is that one pays net and the other side recovers the tax from HMRC. Remember that currently the maximum that can be contributed, including any by your employer, may not exceed 40K in the current tax year.
The amount to be entered on your tax return is your personal contributions paid to the scheme, plus the basic rate tax relief.
I do hope that I have resolved your problem, Morten.
No, you can claim your net contribution plus the basic rate of tax. It will all adjust out correctly in the end as HMRC will have your P60 filed. You are entitled to claim the amount I told you and the tax will be adjusted automatically.