Exactly as @Choux says :)
If those were your only sources of income and you didn’t have any interest, capital gains, dividend income or other pension contributions then your total income was £64,500. Minus £18,750 pension contribution leaves a taxable income of £45,750.
Assuming a standard tax code then the tax works out as:
£12,570 @ 0%
£45,750 - £18,750 at 20%, which is £6,636
Nothing taxed at 40% as you are below the threshold
But you will have paid
£12,570 at 0%
£50,270 - £12,570 at 20%, which is £7,540
£64,500 - £50,270 at 40%, which is £5,692
Total tax paid of £13,232
So you are due a refund of £6,596, less the £3,750 tax relief already credited to your SIPP, which is £2,846. Less than the £3,750 credited to your SIPP as not all of it gets relief at 40% because the contribution takes you below the 40% threshold by about £5k.
When you complete the SA online you’ll be asked where you want any refund to be sent, and can enter your bank details. Once your self assessment refund has been processed you should get the money in a few weeks…but in my experience it can take much longer if this is a one off than if you do it each year!
So you get £18,750 added to your pension for £18,750 - £3,750 - £2,846, which is £12,154. As a check, you paid £15,000 cash into the account but that should have cost you £12,154. One minus the other is £2,846, which is the tax refund due.