The Ashes: Joe Root jokes, brother Billy nastier than Shane Watson and co.

England opener Joe Root revealed a barrage of good-natured abuse from his brother inspired him to destroy Australia with a majestic 178 not out on the third day of the second Test at Lord's on Saturday.

At 22 years and 202 days, Root became England's youngest Ashes centurion at Lord's as the Yorkshire batsman recorded his second ton in just his eighth Test.

Root's heroics provided the foundation for England to push Australia closer to a second successive defeat, with the hosts losing only two wickets to close on 333 for five, a lead of 566 with two days to play.

It was even sweeter for Root as his younger brother Billy, a member of the ground staff of Marylebone Cricket Club (MCC), which owns Lord's, served as England's 12th man and drinks carrier for much of the day.

That meant England's star performer was subjected to some good natured jibes from Billy during every drinks break.

"He has been abusing me all day bringing drinks out, so he actually had to be nice to me when I got the hundred," Root said.

"He was telling me I was scoring too slowly and saying he would have been smashing it to all parts.

"To be honest it was nastier than anything said to me by Shane Watson and the Australian bowlers.

"It was good to share it with my family though. My parents were here as well and I'm sure they enjoyed it."

Root, who scored his maiden Test century against New Zealand on home soil at Headingley in May, had only mustered 41 runs in three previous innings in the series, but he seized his chance to grind Australia into the dust in sensational fashion.

"It would be wrong to say you don't get nervous but you just try to enjoy it more than anything," Root said.

"It's something I've always wanted to do. It would be silly to ruin it by being anxious. It was great. I really enjoyed it.

"Headingley was quite special but to do it at Lord's against Australia is very enjoyable.

"I watched the Ashes when I was a boy and it's great to be part of it now."

With England's lead already well in excess of the highest successful fourth-innings chase in Test history, captain Alastair Cook must decide whether to declare overnight and give himself two full days to bowl Australia out, or bat on.

Australia pace bowler Peter Siddle expects England to keep batting for at least the first hour on Sunday.

But whenever Cook's declaration eventually comes, Siddle insists Australia can still hold on for a draw.

"I'd like to think we can bat for two day if needed. If we play good patient cricket we can draw this match," Siddle said.

"However long we have to bat for that's what we have to do."

England's Graeme Swann took five wickets for 44 runs in Australia's woeful first innings 128 and the off-spinner is likely to be even more effective on a wearing pitch.

"We know we have to bat better against Swann than we did in the first innings, but the batters will be up for it," Siddle said.

Siddle refused to criticise Ian Bell after the England batsman opted not to walk when he appeared to edge Ryan Harris to Steve Smith in the gully before tea.

Third umpire Tony Hill reviewed the incident on replay and gave Bell not out.

"He (Smith) wasn't quite sure and, as everyone does, you leave it to the umpire," Siddle said.

"It went upstairs and obviously it's not out."