Lewis put the finishing touch to a wonderful team move late in the first-half before he profited from good work by Kion Etete and then smashed home a penalty to complete his hat-trick. Here are our player ratings from Town's fourth successive victory...
1. Liam Roberts
Now over 400 minutes since his goal has been breached. He's not even had a save to make in the last two games with both Tranmere and Carlisle failing to register a single shot on target. So well protected by those in front and always alert when crosses or high balls come into his box.... 7.5
2. Aaron McGowan
Did not look right from a very early stage and despite his determination to stay on he eventually succumbed to injury. Ends his run of playing every minute in the league this season. Has two weeks to recover until the next league game... 6
3. Fraser Horsfall
After a rocky opening 10 minutes when Cobblers survived a couple of scares, this was an extremely comfortable day at the office for Town's two centre-backs. Carlisle's forward players were given absolutely nothing to go on... 7.5
4. Jon Guthrie
Won his headers, made his interceptions and successfully snuffed out the danger on the very rare occasions a Carlisle attack made it beyond halfway... 7.5