If only there was a Chartered Tax Adviser on here...
The purpose of the salary sacrifice is that your 'contribution' comes out of your gross salary in return for your employer making the contributions.
So, technically you haven't made any contributions.
The tax position for you is broadly the same as you have effectively had tax and NIC relief already. The employer saves the employer NIC. Such arrangements used to get colloquially termed "NIC save".
Generally that NIC save helps employers to fund a better contribution.