Manchester United guaranteed Sir Alex Ferguson a good night's sleep by maintaining their six-point lead at the Barclays Premiership summit with an Old Trafford victory stroll against rock-bottom Watford.

In the aftermath of the gut-wrenching defeat at Arsenal 10 days ago, Ferguson admitted he 'did not sleep a wink' as he mulled over the manner of the last-gasp loss.

But there will be no need for the Scot to burn the midnight oil in his Wilmslow home again as United roused themselves after a lethargic start to eventually romp home, extending their goal difference advantage over Chelsea to 14 in the process.

After Cristiano Ronaldo had slammed the hosts in front with his 14th goal of the season and Lloyd Doyley had doubled their lead with an unfortunate own goal, Henrik Larsson and Wayne Rooney also found the target as United recorded their biggest win since a similar sized success at Bolton three months ago.

Ferguson had already made it clear he intends to fully utilise his squad over the crucial final third of the campaign so - with a far more testing visit to Tottenham looming on Sunday - it was no surprise Ryan Giggs, Paul Scholes, Edwin van der Sar and Larsson were all omitted from the hosts' starting line-up.

Kieran Richardson was one of the men given a rare opportunity but he did not make much of a case for more regular appearances in a lacklustre start which epitomised the general United effort.

In pulling 10 men behind the ball at every opportunity, Watford succeeded in denying their hosts space and their goal had barely been threatened when Jay Demerit instinctively tugged Ole Gunnar Solskjaer's shirt as the Norwegian attempted to reach Rooney's lay-off.

Thankfully for United, eagle-eyed referee Mike Dean spotted the infringement and up stepped Ronaldo to slam the ball past Richard Lee.

The goal hardly served to send an extra sense of urgency through the home ranks and it was more through luck than judgement that they were not pegged back when Tomasz Kuszczak misread his angles as he attempted to turn Chris Powell's cross over the bar and instead pushed it onto his own post.

It was probably the shock United needed as their attacking play suddenly did become more incisive.

With Ronaldo and Rooney - both restored to Ferguson's starting line-up - growing in influence, so the Red Devils started to carve out an increasing number of chances.

Nemanja Vidic was not far off with two headers from Michael Carrick corners and Richardson saw his shot flash onto the roof of Watford's net.

The Red Devils maintained their focus after the break too, with Rooney and Ronaldo both going close before Richardson curled over the cross which Doyley, with Ronaldo breathing down his neck, stuck out his head to nod into his own net.

To all intents and purposes, it was game over. But United, mindful of how close the title battle could turn out to be, went for the jugular.

Lee had already pulled off one stupendous save to prevent Adrian Mariappa scoring a second own goal before the Red Devils really cut loose.

Five minutes after his introduction for Solskjaer, Larsson laid off a pass to Rooney, then sped onto a clinical through ball from the England man before rolling a shot into the corner.

United bagged another with their next attack, a product of an outrageous double act between Ronaldo and Rooney.

The Portugal man started it with a searing run across the visitors' box, then amazingly chipping the ball over Watford's startled defence to Rooney, who promptly launched his shot high over Lee before watching it drop into the goal.

Although it must have been tempting to inflict further damage on a Watford side whose two-game winning run was brought to an abrupt halt.

Instead, Ferguson offered Wes Brown the opportunity to replace Carrick in central midfield.

It was hardly surprising some of the fire went out of United's play after that, although only the base of a post denied a Ronaldo shot which would have completed their biggest win of the campaign.

Teams

Man Utd Kuszczak, Neville, Ferdinand, Vidic (Silvestre 75),Heinze, Ronaldo, O'Shea, Carrick (Brown 75), Richardson,Rooney, Solskjaer (Larsson 66).

Subs Not Used: Van der Sar, Park.

Booked: Vidic, Heinze.

Goals: Ronaldo 20 pen, Doyley 61 og, Larsson 70, Rooney 71.

Watford Lee, Mariappa, Doyley, DeMerit, Stewart,Smith (Williamson 72), Bangura, Francis, Powell (Hoskins 61),Bouazza, Henderson (Kabba 75).

Subs Not Used: Chamberlain, McNamee.

Booked: DeMerit, Henderson.

Att: 76,032

Ref: M Dean (Wirral).