United ran out 3-0 winners on Saturday, with Obi-Martin scoring a fourth goal of the season for his new club.
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Jim Thwaites and Benito Mandato also were on the scoresheet for United.
Former Arsenal striker Obi-Martin had also scored a hat-trick against Nottingham Forest last month.
United U18s are top of the Premier League North table, having won all nine of their fixtures.