BRITANNIA Building society is offering an investment account that guarantees investors 150 per cent of the growth in the FT-SE 100 share index over three years. If the FT-SE 100 falls, investors will receive their original investment back. The minimum investment in the Equity Linked Account is pounds 1,000 and the maximum is pounds 500,000. The account opens on Budget day, 16 March, and closes on 23 April. No withdrawals are permitted before maturity.