At the start of the quarter, Poly recovered from 6-4 down to level at 6-6 . But then Manchester hit a purple patch, scoring four in double-quick time through Conor Lynch, Harry Dunkerley, Eleftheios Chelioudakis and Dan Padden to build a 10-6 lead.
It was a deficit from which Poly were never able to recover, despite rallying strongly in the final stages.
Earlier, Welsh Wanderers took third place, defeating Sutton & Cheam 16-7.