We had (and have still to an extent) this problem with Fudge. We developed 'What's This Fudge?' by asking the question whenever we came across a toy in the garden or a stick outside (she loves sticks of all kinds) and she learnt that when we siad that it meant 'I've got something interesting and I'll throw it if you come here and sit'. Her deal was that she would come close enough to us but not so we could get her. 'What's This' works really well for us- as does Pammy's tip of not always putting her on the lead. Also- if you can- try putting the dog on the lead for a short while then releasing him. This way he learns that the lead doesn't mean the end of the walk.
Yes that works well if they're in earshot or whistles (I use a gundog one) if too far or if its a windy day.
One really good thing to do if you grab their attention is to then begin running away from them which works a treat as they get scared you might leave them. Even if mine are coming back on a recall I will still walk slowly backwards as I do it just in case one of them decides to change their minds (which knowing cockers is possible).
Try recall exercises/games and basically call them back often even when close to you as if you just do it when you want them back on the lead then they'll work that one out very quickly. The idea is to call them back shortly after they go off lead and either give them a treat or play a very enthusiastic game with them. The idea is to be as enthusiatic as possible to the point of making a complete fool of yourself as often this enthusiasm is infectious to a cocker as they want to know what they're missing out on.
Practising recall is the key wherever, whenever, and use variety so they don't sus out what comes next.
Hide and seek games are also a good way to keep them from running off, mine stay not too far away form me if they think you're going to disappear.
Never chase your cocker as they'll either think its part of a really good game or that you want to go that way anyway and feel they have to get there 1st.
Even if mine are just about to run into danger I can see ahead I run away from them (making sure they know I'm doing it by jumping up and down and basically being very loud and then either run backwards or just as far away form them as you can). As I was going uder a road bridge recenly Wilf recently left my side momentarily and slipped up the steps (this is a busy 'A' road), I shouted very loudly at him to stop which he did but you could see that if I didn't do anything quickly then seconds later he'd just carry on going, but instinctively I ran backwards and he quickly followed.
A friend can never get his cocker back when he wants, but I can, mainly because he's so reserved about calling her back each time while I become enthusiastically embarrassing and it works every time with her (may be she just wants me to shut up LOL).