Monthly income bonds are fine if you are looking for an income, but if your looking for capital growth then your money can be working a lot harder for you elsewhere.
If you are looking to invest for the term stated then you could look at an investment fund that invests in assets such as bonds and equities rather than looking at deposit based or interest rate baring accounts such as ns&i.
You dont have to take risks with your capital as some investment guarantee your capital at maturity. ns&i have such an investment, however better ones can be found with some high street banks. The Abbey have some Guaranteed Bonds that could provide a much better return than ns&i but also keeping your capital safe.
Alternatively, if you have a more risky approach to the returns your investment could provide, then you could look at a managed fund approach and by doing so utilise your ISA allowances ove rsubsequent tax years (�7k pa). The value of these types of finds will fluctuate daily, but coudl also provide you with, potentially a much greater return than those of ns&i or deposit based accounts such as income bonds.
With regard to your money being safe by investing it in ns&i products. The fact that they are government based is true, however if you set up an alternative investment, especially through a bank, your money is covered By the Financial Services Compensation Scheme, which basically guaranteed that should the product provider (the bank/investment comapny) not be able to meet their obligations to you (i.e go bust) then the first �30,000 is guaranteed. So as you are investing �20k, you are covered by the scheme.