Diversification advice

I am looking at opening a Stocks & Shares ISA (a tax wrapper that is exempt from both capital gains and dividends taxes) with Vanguard, and am not sure whether to invest in an ETF that attempts to mirror something like the S&P 500, where I’m effectively betting that the US economy will perform well in the long run, or a cheap global tracker fund, which should include plenty of US equities, so assuming that I cannot accurately predict long term trends in the global markets, should I opt for one of these instead to spread my bets more effectively?

You’ve sort of answered your own question! But yes you should diversify as much as possible, putting it all in one country’s market would generally be a bad idea.

Ideally you want a mixture of bonds and shares, the Vanguard managed funds are pretty good.

Take a look at the Vanguard LifeStrategy funds. They are a mixture of bonds and equities, e.g. LifeStrategy 80 is 80% equities, 20% bonds.

Bonds haven’t faired particularly well recently but their future looks reasonable. There’s also a LifeStrategy 100 if you want just equities.