Just wondered if you've tried doing a stint of lead training which isn't actually part of a walk but is purely "training"? I've been doing this recently with Humphrey and am finding its starting to have some effect, he still pulls when we first leave the house and he's excited but after a few minutes I'm finding that he's responding better to the heel command.
I've found a field thats more or less unfrequented, so there are very few smells and distractions. I let him let off a bit of steam for a bit and then we do ten or so minutes of solid heel walking on lead, I continually change direction about every 15 or 20 paces so he has to keep an eye on me and I also change speed from a walk, to jog or very very slow and he should sit as soon as I stop. Its really just classic basic training school stuff but I'm finding it much easier to be consistant on a field, on our own when its just him and me concentrating on the job in hand than trying to keep myself motivated while trying to just have a carefree walk!
I'm hoping if I can eventually get this perfect without distraction then maybe it'll all start and fall into place in "real life"