My pair of cockers will get away with anything they can, one of the labs pushes his luck as well. Don't let the bugger get their own way and all will be fine. Mine are actually easier off the lead, but people will expect to see them on the lead at times. In short, persevere.
I've recently got a rope type slip lead for our Cocker but we're still learning to walk sensibly.
I think part of the problem with our dog is she is rarely walked on a lead so the training we did previously has gone out the window.