I just want to bump this recipe, as it got me where I wanted to be in terms of brewing a good English bitter. I had an involuntary hiatus from brewing (had a kid, moved, job, etc.), and am now returning. An ESB based on Bob's ratio is the first beer out of the gate, probably the 5th or sixth time I've used his ratio (80% pale malt, 10% Crystal, 10% Sugar, and roughly 0.7 IBU/SG), and that's because this nails the flavor profile and character I want in a good ESB; as close as I can get to a fresh English cask ale without buying a plane ticket.
Anyway, you can tinker with different crystal malts or yeast varieties. I play it a little conservatively and keep everything English, but that's because I love a good English bitter. Stick with the basic ratio, or at least close to it, and you'll be fine.
I suppose my real point is that the ratios Bob laid out here really will serve you well. Of course there's room for creativity and doing your own thing; it's brewing, after all. But this right here is the perfect antidote to the kitchen sink/kid in a candy store methodology that is so tempting when putting together a recipe. Simple and delicious. Thanks, Bob.