If you have a gundog but are not wanting it to fulfill its role as one you have to de-train the gundog from its natural ability, something I personally have issues with, but that is not to say I understand the need on occasions.
If your actively encouraging the dog to hunt while you are walking it ( Off lead walking without a definite alternative task, playing fetch etc ) Then your not helping to de-train the dog from its natural ability and you've created your own problem. If you want the dog to express its natural ability, then your going to have to look at all `the training' aspects of having a working gundog and then mold them to where the rewards are given only to your desired outcome, and that means the dog HAS to be under your control, no ifs buts or maybes.
Taking what you said in your post,
This is your mistake, as your not making the flush the end result and the reward for hunting. In gundog terms yes you want the dog to want to find more birds ( run around after flushing ) or you want the dog steady to flush as the bird may be shot and the dog required (rewarded ) for a retrieve. If you want the dog to see the flush as the end result ( A walking companion/ beating only dog ) then you have to train the dog to stop to flush and/or return to you for reward (recall) without continuing to hunt after the flush ( Unless instructed to do so ). This is why the stop is so important, without it you stand little chance of de-training natural ability of a gundog.However, Mia flushed a hen pheasant about 20 metres away. I expected her to run about a bit, and then come back to me
Finding game less ground can difficult as game moves around, but in general they dislike a lot of disturbance so public parks where a lot of dogs are exercised is a good bet.... but then you have a lot of untrained dogs running about ` only trying to be friendly'
However you can manage your walks by `disturbing' the ground yourself simply by not hunting the dog, not on the way out but on the way back
If you do close control obedience on the way out, not only will you give game a chance to move off, you will also set the framework of control and focus on you in the dogs mind. Hopefully this means when you come back over this same bit of ground and allow the dog to hunt, there will be less of an unexpected encounter with live game. ( Baring in mind the previous caveat that `game moves'
Hope this gives you something to think about, but like I said I'm probably not the best person to ask for advise on de-training a gundog as I'll always side with the dog. I often find they have a better outlook on life than the monkeys on the other end of the lead