NSI Index Linked certificates income or capital

We have a client who has index linked NSI certificates. The interest rate is 0.01% but with the index linking adjustment gives an effective higher rate of return at over 3%. For the purposes of gifts out of income and subject to the other rules that apply to such gifts being met how much of the value can be deemed income? Is the increase in the value of the certificate, which is in line with inflation capital or income?

Frances White
Number One Legal