Chris Forrester glanced home a late winner to secure Posh a derby day success.Peterborough United secured a derby day success at Sixfields as Chris Forrester glanced home a stoppage time header to secure a fifth successive win in all competitions for Grant McCann's men.
Posh created one or two openings from long range inside the first few minutes at Sixfields with midfield pair Chris Forrester and Leonardo Da Silva Lopes both firing over.
Da Silva Lopes saw a free-kick gathered at the second attempt by Adam Smith on eight minutes after Coulthirst was felled 35-yards from goal.
Northampton had a penalty appeal on 13 minutes as Da Silva Lopes made a challenge in the box on Hoskins, the referee waving appeals away.
Nichols was a whisker away at the other end after cutting inside onto his right-foot 30-yards from goal, inches wide from the ex-Exeter man.
Taylor sent a free-kick straight into the wall on 18 minutes after a foul by Forrester. Diamond saw a looping header comfortably claimed by McGee on 21 minutes after a decent ball into the box from O'Toole.
Gabriel Zakuani replaced the injured Diamond on 23 minutes, receiving a decent ovation from the Posh fans behind the goal.
Revell headed over a corner kick moments later as Town continued to probe. Bostwick was cautioned for a shirt tug on 27 minutes.
McGee made a double stop to deny Moloney and then Revell on the half hour mark, the second save was an excellent one with his legs to divert the effort around the post.
Bostwick planted a header inches past the post from a decent Maddison delivery on 36 minutes. Da Silva Lopes was denied by Smith moments later although it was routine for the goalkeeper.
Bostwick picked out the run of Maddison on 42 minutes, but the former Newcastle and Gateshead man was off balance when he let fly and the shot cleared the crossbar.
Bostwick looped a header from a corner kick wide of the target after Coulthirst dithered with others making great strides along side him following a swift counter attack.
Taylor fired wide of the upright for the Cobblers on 62 minutes after cutting inside onto his favoured left-foot.
Nyatanga produced a fine block to deny Forrester before Maddison saw a shot blocked. Posh made a double change on 68 minutes with the strikers being replaced, Lee Angol and Paul Taylor the men to be introduced from the sidelines.
Angol tested Smith with a curler from range after linking well with Taylor on 75 minutes. Posh introduced Gwion Edwards for Da Silva Lopes with Beautyman replacing McCourt for the hosts.
On 81 minutes, McGee produced a superb save to tip over a curling effort from Matty Taylor. Hoskins saw a deflected effort clip the top of the crossbar as we entered two minutes of added time at Sixfields.
In the last minute of the game, Posh snatched all three points as a superb cross from Maddison was met by FORRESTER who glanced home to secure a fifth successive win in all competitions.
Northampton: Smith, Maloney, Anderson, Buchanan, Diamond (sub Zakuani 23min), Revell, McCourt (sub Beautyman 78min), Taylor, Hoskins, O'Toole, Nyatanga. Unused subs: Richards, Gorre, Potter, Hanley, Cornell.
Posh: McGee, Hughes, Smith, Baldwin, Tafazolli, Bostwick, Forrester, Da Silva Lopes (sub Edwards 80min), Coulthirst (sub Angol 68min), Nichols (sub Taylor 68min), Maddison. Unused subs: Tyler, White, Binnom-Williams, Moncur.
Half-time: Northampton 0-0 Peterborough United
Full-time: Northampton 0-1 Peterborough United
Attendance: 7,675 (1,462 Posh fans)