When to use 'fewer' rather than 'less'?

It looks like there are grammar vigilantes all over the world!

"Tesco is changing its checkout signs after coming under criticism from linguists for using "less" rather than "fewer"" says the BBC .... and
"Tesco is replacing its current "10 items or less" fast-track checkout notices with signs saying "Up to 10 items"."

The Plain English Campaign has a simple rule of thumb to help everyone: less means "not as much," whereas fewer means "not as many".