WESTBURY, led superbly by defender and captain Paul Beavers, advanced to the quarter finals with three goals in 17 second-half minutes.
Goalkeeper Lewis Porter and striker Nathan Hallett were also in top form while the rest of the team contributed to the performance.
Bassett, riding high in the Hellenic League Division One West, had the better of the opening period with United keeper Lewis Porter making several fine saves from Matt Bown, Steve Yeardley and Ethan Delaney twice.
Other openings were denied by strong work by Beavers, Ryan Baggs and Luke Stanley.
At the other end efforts by the home side were off target with Hallett twice narrowly missing the target, although Matt Mead did make Luke Sharps make a save.
Hallett put Westbury on the way to victory with an excellent low 30-yard shot on 50 minutes and he then created the second goal on the hour when a bursting run was ended by a fierce challenge and a free kick awarded.
Hallett dusted himself off and struck the 20-yard free kick which was blocked and the ball broke to Joe Kirkpatrick who showed good composure to finish from inside the box.
The visitors had a few chances to get back into the game with Bown missing the target and adam Corcoran having an effort blocked in the six-yard box but they were outdone by an audacious Mark Young goal on 67 minutes.
Young received the ball wide on the right wing and sent a swerving ball beyond the keeper and into the net.
MORE FOOTBALL REPORTS, REACTION & PICTURES IN THURSDAY'S GAZETTE & HERALD AND FRIDAY'S WILTSHIRE TIMES