Two penalties proved their undoing as they suffered a fruitless away day.
Andre James put the home side ahead from the spot after 51 minutes.
And Diamonds cause became even harder just five minutes later when Callum Westwood handled in the area and was given his marching orders by the referee.
James’ penalty was saved but he put home the rebound to double the lead.
Nabil Shariff got Diamonds back in the game with six minutes remaining but, although they had chances to equalise, they were unable to grab another vital goal.
Corby Town claimed another point in their bid for Evo-Stik Northern Premier League survival as they drew 0-0 with Marine at Steel Park.
It was an important result for the Steelmen having lost two games on the bounce.
In the Southern League Premier Division, Kettering Town were given a fright by bottom-of-the-table Cinderford Town before eventually running out 3-1 winners.
Cinderford, who have won just seven games all season, led 1-0 at half-time thanks to Jarrad Welch effort on 41 minutes.
The Poppies toiled away until 75 minutes had gone when James Brighton finally got them back on level terms.
Aaron O’Connor put them ahead on 81 minutes before Ben Baker made the game safe with five minutes left.