The 18-year-old forward finished as the City’s under-18`s top scorer at the conclusion of the 2016/17 campaign.
Allen could make his debut tomorrow at home to Larkhall Athletic.
Meanwhile, Steve Davies has decided to leave the club and has signed for local league rivals Yate Town.
Davies returned to the club during the summer of 2017 from Shortwood United.
He started his career with Swansea City, then onto Hartpury, Forest Green Rovers and Gloucester City.
The experienced wide man left Mangotsfield in November 2015 for North Leigh, after fifty-eight appearances and scoring 10 goals.
He has also won a Southern League title with Cirencester Town.
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