pup keeps barking at the cat

[Dieu]

Hi. We've had our cat (originally a rescue) for nearly 5 years, and our Shih Tzu pup is almost 11 months old. So both pets have been cohabiting for 8 months or so. It hasn't gone too badly; Bruce the cat is boss as he's older and was obviously here first. He tolerates the dog, as Teddy is fairly small and unaggressive. They can be in the same room together, and Teddy will get swiped if he gets too close, which is fair enough. It is never going to be a match made in heaven, as our cat is naturally just wary of him.
Recently however, the situation has been driving me nuts. The cat will be sleeping on a bed, minding his own business, and Teddy will seek him out and just start barking at him. The cat looks like he couldn't care less, but it's bloody annoying as we're in a flat and I have always been a considerate neighbour when it comes to noise levels in particular. I have no doubt that pup wants to play, but at nearly 11 months, he has to now get the message that this 'aint going to happen!
I keep them apart as much as I can, (and obviously remove pup from the situation when he's making a nuisance of himself) but not all the doors in this place close properly, so Teddy can more often than not find his way in. I always make sure they are apart if I have to go out.
Any tips? I am getting royally pissed off with the constant sound of cat growling/spitting/hissing and puppy barking. In all honesty, I am more bonded with the pup (who is great in every other way!) as I'm less of a cat person, but I obviously find myself being protective of the cat as he hasn't done anything to warrant this. I do sometimes wish he was a bit more accepting and less disapproving though, as pup is doing it to get a rise out of him, and usually succeeds.
Hopefully things will settle again once Teddy exits the teenage phase ...
Thanks everyone.