Bancroft gave their survival chances in London Three North East a shot in the arm with a crucial bonus-point win over long-standing rivals Campion at the weekend, whom they had failed to overcome in four previous meetings.
It looked as if the drought would continue for the Bs, as Campion were encamped in the hosts' 22 for much of the early exchanges.
However, the defence held firm and a breakaway try gave them the lead. Alex Silk led the charge and the ball was eventually worked to Glenn Adams to cross over, with Adam Capon adding the extras.
One try soon became two as the home side immediately put the visitors under the cosh from the restart. A series of short bursts resulted in Francois Nel crashing over for an unconverted score.
Bancroft refused to let up after the break, cantering into a 21-0 lead with Ricardo Dubbery rushing 60 yards for the third try, which was converted by Capon.
Campion responded superbly with a dominant final 20-minute display that included three tries, bringing them to within four points of the hosts.
However, Jordan Wigham surged over for the decisive score for the bonus point, leaving the Bs with a good chance of escaping the drop, should they capitalise in games against their fellow relegation rivals.
It really was a game of two halves at Waltham Way on Saturday, where Chingford saw a 10-point advantage evaporate and overturned by a rampant Sidcup side pushing for promotion in London One North.
Chingford's own promotion aspirations had suffered blow after crushing blow in recent weeks, but it seemed as if they had turned the corner when Kurt Forward struck two penalties and converted his own try for the hosts to lead 13-3 at the turnaround.
But Sidcup proved why they have been one of the strongest teams in London One North with a resurgent display in the second half. Five tries were run in past a bedraggled Chingford defence and, despite a late consolation from Luke Campbell, the visitors emerged 34-18 victors.
Eton Manor, meanwhile, were narrowly defeated 18-14 at Tabard, ending a run of five straight wins for the New Wilderness men.
They remain in the comfort of mid-table with three games of the season to play, Thurrock the visitors on Saturday.
In London Three North East, Wanstead cruised to a 30-5 victory at home to Wisbech. They face three away trips to conclude the season, the first at Upminster this week.
Upper Clapton's extended victory parade in Essex Two continued with a typically emphatic 46-5 win over May & Baker. They bring the curtain down on what they hope will be a perfect campaign at Stanford Le Hope in two weeks.
Ilford Wanderers, meanwhile, have missed out on promotion after a 19-6 loss at Writtle Wanderers. They play Burnham-on-Crouch in their last game on April 9.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here