Jo,
Labs make great small game dogs. And, as usual, their performance in the field comes down to OBedience.
You need to train your dog to "SIT" on whistle (even with lots of distractions). This will allow your dog to eventually learn to "Sit to flush, Sit to shot, and Sit to fall". Then when a rabbit flushes, you blow the SIT whistle and the dog should obey.