If you are a higher rate taxpayer and make pension contributions into a SIPP you can get the tax back at 40% (20% given at source and the rest claimed).
But what if your net income once you've paid into a pension takes you below the 40% tax threshold? E.g. you earn £80k and are able to pay total contributions of £60k. Can you claim 40% on the whole of your contributions or only to the level where you would be a higher rate taxpayer?