Under the current rules, the government adds to your pension contributions at the 20% basic rate. For instance, if you save £4,000 in a personal pension, the government tops this up to £5,000. If you are a higher rate taxpayer there is a further £1,000 tax relief when your tax liability is calculated, reducing the net cost to £3,000.
Additional pension contributions can be even more effective if your income is between £100,000 and £125,140 as the gross pension contribution reduces net income for the purposes of the reduction in the personal allowance. Note that for every £2 of income in excess of £100,000, the £12,570 personal allowance is reduced by £1, with reduction to nil where net income is £125,140 or more. This is effectively a 60% tax saving.