Hello Leanne,
sorry we missed your post, how did things turn out in the end?
In answer to your question, I can tell you what I do now, but I can't relate it to when my ds was the age of your lo, b/c he was on nowhere near as good a routine as yours, I'm so impressed that you've got him on a good routine already
Basically, I work on the premis that if he sleeps over on his nap time, there's usually a reason. If your lo had a huge meltdown, then he probably exhausted himself, and needs to catch up. If he goes over, say 5-10 minutes, I don't worry about it, but if it's much more than 1/2 an hour, then I will start to make soft noises outside his room, so that he stirs awake on his own, without me actually having to get him up. I don't leave it much more than that, b/c then that's setting up problems in the routine for the rest of the day.
HTH