Sadlier struck in the 37th minute to give the Londoners a two-goal aggregate victory after they had won the first leg 1-0 at the New Den. Even though Siggy Eyjolfsson pulled a goal back with a minute left it was too late for the home side to revive their hopes.
Millwall must wait to see who they play on their big day. Wigan will take a 2-0 lead into the second leg at Wrexham after second-half goals from Kevin Sharp and Stuart Barlow.
The Manchester United youngster Terry Cooke scored a brilliant goal and made another for neighbours City as they clawed their way towards the Second Division play-offs with a 2-1 win over Notts County.
Cooke, who is on loan at Maine Road, fired a stunning low drive to put City two up before the break, having created the first for Michael Brown. But County gave City all sorts of problems, hitting the woodwork three times before finally pulling a goal back after 72 minutes through Mark Stallard.
Bournemouth remained firmly ensconced in the play-off zone after grinding out a goalless draw at Blackpool, while Cardiff missed the chance to leapfrog Cambridge and go back to the top of the Third Division as they lost 1- 0 to a 66th-minute goal from Rotherham's Chris Sedgwick at Millmoor. Elsewhere, Brentford were held to a 1-1 home draw by Halifax despite taking a 28th- minute lead through Scott Partridge. James Stansfield equalised.
The First Division action drew a complete blank, but at least Crewe could glean some consolation from the fact their 0-0 draw with Grimsby took them off the bottom. Bury, meanwhile, have now gone 16 games without a win despite halting Wolverhampton Wanderers' play-off push in another goalless affair.Reuse content