Re not responding and your fear of him running off:
Barnaby is starting to get to the willful teenager stage. If we walk him just one on one he's very well behaved and responsive to the whistle. But if my wife and I, together, take him on the same walk the wilfulness shows up straight away. No reaction to the whistle and he's off exploring far and wide, apparently completely out of control.
We've come to the conclusion that either its because he's not the centre of attention, or most likely as he can keep track of us from the sound of our continuous conversation.
We've been experimenting a little. When he's off hunting in tong grass/ wheat and goes further away from us than he's supposed to, I give him a sharp recall whistle, and then we both crouch down and stay silent....... guess what, he fairly flies back looking for us. This is not an occasional response, it works every time.
Cockers are smart little Bs and will have you running rings if you don't stay one step ahead of them. Behaving unpredictably also's good for retaining their attention. So, if in the nature reserve its safe to do so, when your boy decides to go a-wandering, whistle him a single recall and then hide out of immediate sight and stay completely silent...... with luck he'll come looking for you very quickly.
Good luck