My suggestion is that you should avoid anything complicated. I don't understand "once the money matured with the FTSE index, it stayed there" so I personally would avoid that. I would, however, happily invest in a stock market investment that put 5% of the fund in one named equity, 4% in another, and so on. When those shares went up or down, I would expect my investment to do the same.
It sounds like you may be uncomfortable about investing in something that might go down, as many people are, so you should stick to cash savings accounts and fixed term cash bonds with banks and building societies that you know and trust.
There are many types of bonds but fixed term cash bonds are very different to investment bonds.