A battling display from Diss was not enough to prevent leaders Bury from recording their 14th consecutive win of the season in London 1N.
Diss conceded a penalty and had Fraser Hall sin-binned early on, but Chris Beaird levelled the scores.
Bury led 6-3 at half-time after converting another penalty, while Beaird was off target with a penalty for the visitors.
Beaird made amends with two penalties in the second half to give Diss a 9-6 lead, while Bury had a man sent to the sin bin.
But the hosts scored a converted try with around 20 minutes remaining and soon afterwards crossed the line again – this time not converted – to take a decisive lead.
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