The main thing here is to be 100% certain whether your pension payments have been deducted from gross salary (no extra relief due as taxable pay already reduced) - or from your net salary (extra relief available on grossed up value of your contributions if you are higher rate taxpayer). Some of the technical terms used by pension providers are somewhat confusing to say the least!
The best way to ensure you know without and doubts to get confirmation in writing from your employer as to how scheme is meant to operate and check that this tallies with the amounts that show up on your annual pension statement.
I would expect it to be clear on the annual pension statement whether your contributions are exactly as deducted from your gross salary - or whether hmrc have added 25% of this amount into your pension pot if the deductions are deducted from net pay.
Note your p60 year end taxable pay total should have lower total (after deduction of pension payments) if you have pension payments deducted from gross salary.
You only enter contributions made out of your net salary. As all your contributions are made via your employment it is likely that you obtain tax relief at source (i.e. they deduct the pension contributions before calculating your tax) so no additional tax relief is due so you wouldn't put anything on your tax return.
Note the bit in bold mentioned by Adam ref obtaining tax relief at source - i would add that its all a bit nonsensical but the term "relief at source" when used by pension providers in relation to deductions indicates that the payments have been deducted from net pay not gross pay !! even though Adam is 100% correct that deductions from gross pay mean you have already obtained tax relief at source in full!! - They really could not have chosen a worse way to officially use the term "relief at source" as it is the probably? the exact opposite of what you would expect it to mean!!
To add to the comedy value the method of deduction from gross salary is technically called "net pay arrangement" - so again probably?? the opposite of what you would expect.
To add to the confusion some third party operators normally do deduction from net salary (nest normally go down this route) and some normally deduct from gross salary (peoples pension normally go down this route) - so i would never presume its one method or the other without checking the full details!