Sounds like they aren't investing very well for themselves since the SP 500 is up 150-350% from 2009 depending on the dates.

Is it the same 1000 families from 2009, the article infers that it's the same 1000 families, whereas in reality there will be some people who drop out and some who come in.

Just like backward testing in stock, if you're measuring against the top X companies in 2000, they are not the same top 100 companies in 2018.

Slightly disingenuous from the Grauniad - whilst they might have twice as many pounds on paper, the real world value of 2018 pounds is significantly lower than 2006 pounds. A PPP measure would be more accurate.

Anyone who keeps wealth in the stock market has done the same.

The stock market is not without risk either. What is double gains now can be 50% losses next.