Hi Mart.07
In your example you are paying £6000 into your pension. As long as your income above the higher rate tax threshold covers the £6000 you can claim relief on the full contributions.
The pension provider will claim 20% relief giving the figure £6000 x100/80 = £7500. £7500 at 20% £1500.
As you are higher rate an additional 20% would be due. As you are in Self Assessment your basic rate tax band is increased by 7500 to give you the additional 20% relief £1500.
More information can be found online
Tax on your private pension contributions
Thank you