Goals from Colby Bishop – one of which was a penalty – either side of half-time secured the Blues their first win of the new League One season.
It maintains Danny Cowley’s side’s unbeaten start to the season and moves Pompey up to ninth.
Here’s how The News rated the performance as temperatures rose to 33 degrees in Gloucestershire.
1. Josh Griffiths - 7
Did what was asked of him. Can't do more than that. Photo: NationalWorld
2. Joe Rafferty - 8
Another solid performance from the right-back as he proves a shrewd piece of business. Photo: NationalWorld
3. Sean Raggett - 9
Had a real tussle with on-loan Saints striker Dan Nlundulu - but there was only going to be one winner. Photo: NationalWorld
4. Michael Morrison - 8
Proved rock solid in the heart of the back line and set Bishop for his opener. Photo: NationalWorld