I never met a dog that didn't respond to food. Start with long leads and a pocket full of (small) biscuits. Let them stretch their leads to the fullest, call them back and give them a biscuit. Do it again, at random intervals between 30 seconds and ten minutes over the course of a four or five hour walk. Next time you're out, only give them a biscuit every other call back. Then one in three, etc. And tone of voice is important; low, quick and growled is more effective and they won't see you as aggressive (although other walkers might), just giving a command. Spaniels love to work and to be directed. It's also useful to be able to speak to more than one of them at a time. When I call mine "dogs", they know I mean all of them.
I have three cockers (albeit working dogs from good stock which I would have been disappointed if they didn't behave) and I can just about call all of them back every time with only this very basic training. They don't bother with sheep or other livestock, and I rarely if ever have to put them on a lead at all these days.. except when, for example, parking on a layby next to the A9
When I reached the stage when farmers and landowners would happily drive straight past me on quads with the dogs sat next to me leadless and still waiting to be told to move, I knew I had made it. Took a while, but was worth it. Although I suspect that the bloke whose coffee Lola drank on top of Blencathra has still not forgiven her