Ian, - I think your p.s. sums it up, - I always remind myself of that old prayer about having the courage to change things that can be changed but having the grace to accept those that can't and I think a great majority of cockers fall into that last catagory!!!!
Incidently, a tip I got from one of Zak George's videos, which has worked a few times when Humph is being particularly difficult to control, is to stop and do something they CAN do well, - so I just do a stop and sit or down occasionally while we're walking, which does seem to calm things down a little and gets him back into "listen with to mother" modus!!