I had the same problem!! Luke would always only sleep 45 minutes (pretty much on the dot) and then wake... In the end I just had to wait it out... even now he sleeps anywhere between 45 minutes and 2 hours... and I can't bank on which it will be when. Today he had only 2 45 minute sleeps...
Sorry that doesn't help much.
Oh yeah, I tried leaving him in bed, replacing dummy etc and sometimes could leave him there for up to an hour (before he started crying), but he wouldn't go back to sleep - gave up in the end...