Thanks to the various recommendations on this thread & other similar ones, I've also gone for longer term S&S ISA investment for the first time.
In the past I've been very 'safe' on investments, slowly building up to the maximim holding in Premium Bonds which I'll keep as easily convertible into cash with the occasional bit of prize money for trivial spending. I've also only ever gone for the one year savings bonds and got lucky last year when the rate dropped by 0.5% the day after I took one out.
Having had a look around and, with the best bond rate being only 0.6% for one year, I've decided to take the risk & put the full £20,000 allowance into a S&S ISA with Vanguard, but true to my cautious nature it's been split with £10k each in a 40/60 & a 60/40 shares/bonds fund. I'm not planning to touch this for about 5 years other than to possibly add to it each year, with the aim of having a bit of an extra lump sum paying out part way though my planned early retirement.