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I have done the sums (which I was avoiding), assuming an inflation rate of 3%. The index-linked figure will always be worth £770 in today's terms, so after 20 years she will have received £15400 in today's terms. The flat rate figure will be worth £833.09 in 20 years time, but the sum of 20 terms with a common ratio of 0.97 is £23,296.90. In 30 years the figures are £23,100 and £30,588.57 respectively. It's a no-brainer.