Potty training success?

We're getting on very well with potty training. Almost every wee at least makes it to the toilet. Unfortunately we don't seem to have quite reached the stage where Harry tells us that he needs to go, but as long as we put him on the toilet frequently we can manage long car journeys, day trips and so on. Now that the novelty has worn off it can be a bit of a battle to get him to sit on the toilet though. We end up performing a pantomime which goes:

Me - Harry, do you need a wee?

Harry - No

Me - Harry, do you need to sit on the toilet!
Harry - NO! Waaaa!

Me - Harry, would you like a biscuit?

Harry - Yes!

Me - So what do you need to do first?

Harry - Sit on the toilet (heads for toilet). Mummy, I need the toilet, I tell you!

Me - Well you didn't really tell me did you?

Harry - I tell you all the time! (He does what is expected of him on the toilet) That was a really really big one! I have a biscuit now!

If I try to test him and not bribe him to sit on the toilet, he will invariably have an accident. Just hoping that this is just a phase and that he'll work it out soon!