I had a look and have found it here:
If you're responsible for at least one child
You'll still qualify for Working Tax Credit if either of the following applies:
you're single and working at least 16 hours a week
you're in a couple, and meet the new hours rules for couples - see the section above
The 24 hours is for couples and says
"Couples with children - new working hours rules for Working Tax Credit
At the moment, if you're responsible for at least one child and working at least 16 hours a week, you can get Working Tax Credit.
From 6 April 2012, the rules for couples with at least one child are changing. In most cases, to qualify for Working Tax Credit your joint working hours will need to be at least 24 a week.
This will mean:
if you both work your joint weekly hours must be at least 24, with one of you working at least 16 hours a week
if only one of you works, that person must be working at least 24 hours a week
If neither of these apply, your Working Tax Credit will stop from 6 April 2012. But there are some exceptions to the new rules - see the sections listed just below.
You can increase the hours you work, so you would still be entitled to Working Tax Credit"
All those things come from here.
So lone parents will still get WTC working 16 hours a week :)