Dean stars with four wickets; Wong takes three wickets on debut
Dunkley hits her first ODI century - off 87 balls
Lamb and Beaumont put on 127 for first wicket
England now lead multi-format series 6-2
Highlights on BBC iPlayer from 07:00 BST on Saturday
Three half-centuries and a century from England's top four have put them in charge of this match.
Sophia Dunkley will grab the headlines, but Tammy Beaumont's 58 set the tone, Emma Lamb continued her fine start to international cricket with 67 and Nat Sciver supported Dunkley with 63 from just 47 deliveries.
South Africa, who looked listless at times in the field, will need to produce something special if they are to win this one.
England's Sophia Dunkley, speaking to Sky Sports: "It was a thrill to get my first century for England. I was over the moon when Heather told me I'd be batting number three. I'm lucky to have this opportunity."
Daniel Norcross
BBC Test Match Special
It's a very well taken catch but that takes nothing away from the mammoth total and the brilliant effort of Sophia Dunkley.
Dunkley c Wolvaardt b de Klerk
Sophia Dunkley goes for the big heave off the final ball but can only hole out to Laura Wolvaardt on the boundary.
Dunkley is dismissed for 107 from 93 balls as the crowd rises in appreciation. A fantastic innings that has put England in a commanding position.
England have reached 337-5 which means South Africa need an imposing 338 to win.
A single off the penultimate ball brings Sophia Dunkley on strike for the final delivery.
That's better. Danni Wyatt digging out a fuller delivery and driving into through point for four.
Danni Wyatt goes for a huge heave but completely misses it and the ball disappears away for four byes.
Trisha Chetty puts her head in her gloves in frustration.
Alex Hartley
Former England bowler on BBC Test Match Special
Already, you sort of feel that England are out of sight for South Africa here. It's going to take a brilliant innings from Wolvaardt or Tryon to be in any contention.
The party tricks are coming out now.
Sophia Dunkley opens the face and plays a lofted ramp shot over the wicketkeeper's head for two.
Seven runs off the penultimate over.
Knight c Mlaba b de Klerk 9 (Eng 318-3)
Heather Knight helps the next ball straight to short fine leg and is out!
Daniel Norcross
BBC Test Match Special
She punches the air with glee! This is her maiden ODI century for Sophia Dunkley. She has had the South Africans running ragged today and has been magnificent.
She has done it! Sophia Dunkley nudges the ball into the on-side for a single to bring up her first ODI century.
Dunkley raises her bat in celebration with a beaming smile on her face.
A wonderful innings with some blistering hitting.
England push for two as Sophia Dunkley moves on to 95.
She then powers a four through the on-side to move on to 99....
Shot of the day. Heck it could be shot of the season from Sophia Dunkley. Outrageous stuff.
She is seven runs away from a century...
A touch of fortune for Heather Knight as she gives herself some room outside off-stump and edges through to where third slip would stand and the ball disappears for four to bring up the England 300.
Shabnim Ismail, who has bowled well, looks a bit peeved.
To make things worse she balls a no ball with the final delivery and has Chloe Tryon to thank to prevent a boundary off the free hit.
Great fielding.
Daniel Norcross
BBC Test Match Special
Image source, Getty ImagesEngland are eyeing 330, maybe a touch more if they can. There is plenty of batting to come.
Skipper Heather Knight is the next batter. What a brilliant time to come into the middle.
Five singles added to the score. Sophia Dunkley is 13 away from a century.
Alex Hartley
Former England bowler on BBC Test Match Special
That's the class of Ismail. It was too full to sweep. It's hit middle stump. A brilliant innings for Natalie Sciver comes to an end.
Sciver b Ismail 63 (Eng 289-2)
Gone! Nat Sciver is bowled attempting a sweep into the on side.
The previous ball she had attempted a ramp shot but Shabnim Ismail had followed her. This time the ball was straighter and clattered into the stumps.
Reward for some good bowling by Ismail. A wonderful innings by Sciver though.
Sophia Dunkley has moved onto 77 which is her best one-day international score.
There's plenty of runs out there too if she wants them.