@cheeseisthebest remind me how old your pup is again sorry?
PoirotPup is 9 months and we did it by first of all doing the flitting game around the house fairly often until he would sometimes stay in one room, or even take himself to a different room to sleep. Don't get me wrong if the babygate to the kitchen is opened he's usually there in a flash, but the regular few minutes of being boring often did help.
Then we identified that he usually had a lovely longish sleep in the morning from 9 - 10.30 pretty reliably most days. He was about 5 months and circumstances meant he needed to be left for 5-10 minutes one day about that time so we tried it and watched him on the camera and he just went to sleep in his usual place. When I came back I sat out in the car for a while to see how he got on and after 30 minutes came in as he was clearly fine.
Then we used to do it a bit more regularly, but making sure we tried to mix up a bit who left in what order and for how long - and putting in lots of fairly short times so we built the knowledge that we always came back. Then we built in some longer periods - where we were at someone's house maybe 5/10 minute drive away and we'd keep an eye let him wake up and make sure we came back before any distress.
Now as long as we have exercised him, fed him, made sure he has gone to toilet we can leave him pretty I'd say for up to three hours (although it is usually closer to 2 max). We put on classic fm to help cut out some of the background noise outside. Make sure everything is safely out of reach etc.. and have the camera's to check on him. If we are going to be somewhere it would take longer than 30 minutes to come back we do have a neighbour who would pop in for an emergency!
I know for some people building it up from the start works, in really small increments, but for us leaving it until he was older, really secure with us and the house I think worked well. We're moving back home soon, so of course it could all go to pot in a new place!!!!!