A friend suggested that i keep him up longer in the morning -she had no idea why but she told me that she did it with her child when she had the same problem and it worked a treat. Especially as later bed times did not seem to help. I started keeping him up till 10am (which was awful the first day - lots of walking around the house pointing things out to keep him distracted) and two days after i did this he started waking at 6:30 and has not looked back. He is now on one sleep at 11:30, and i am working on moving it to 12pm.
If your little one is going down before 12pm, i would try pushing the sleep to 12:00 (or later if you can) and then waking them by 2:30. Apparently after 18 mths they should not sleep past 2:30, or it mucks up bed time. My ds goes down at 7pm. Have no idea if this will help. but it really worked for me.