A convincing away win sees the team progress, despite the hosts three late goals.
The Royals stormed into a three goal lead and extended this to five - nil through after fifty five minutes through, Liam Wilson 2, Marvin Hinds first for the club, Sam Guze and Adam King. However, Mortimer rallied with three goals in a seven minute spell before United's composure saw them into the next round.
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Team selection has not been published for this fixture yet.