5N.I.The present value of a periodic payment under a policy, being a payment in respect of personal injury or disease shall in the case of total permanent incapacity, be such an amount as would, if invested in [F1 the purchase under the Government Annuities Act 1929 [1929 c.29] of a savings bank annuity], purchase an annuity equal to seventy-five per cent. of the annual value of the periodic payment, and, in any other case, shall be such proportion of that amount as may, under the circumstances of the case, be proper.]