I can echo everything that's been said here. We used an extendable lead for Pickle's first few weeks of walks and he is now a wee monkey on the lead, pulling like mad. I think all Cockers will try to pull, but it is something that can be overcome with a lot of patience.
One other thing about the extendable - once or twice Pickle ran at top speed forgetting that he was attached to the lead, and got a nasty jerk to the neck when he got to the end of it. So I would persevere with a normal lead, I think - much safer all round.
Emma and Pickle xx