Yeast is the most important factor in every beer if you want to get serious about your beers, the yeast used determines the beer you produce it is the one thing with the most influence.
Remember this
"Brewers make wort, Yeast make Beer"
Notty or SO4 will give you a good beer but it will be very neutral, and wont have loads of character, if you truely want to brew a yorkshire bitter you need a yorkshire strain of yeast ame with a burton pale ale, or a london porter, or a pilsner lager, each strain will give you different results. but to get the best out of them you need to be pretty good at water profiles to try an mimic the water in their areas, saying that though they will all give good results with a general water profile.
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