If the Johnon controller was set to 68 and you pitched at, say, 75, you fermented way too warm. Cool the wort down below 70, and preferably in the low to mid 60s before pitching for most ale yeasts, and set the controller at 64. You need to count for the exothermic activity of the yeast, which can be 4-6+ degrees over fermentation temp.
And I personally don't care for Nottingham. Try a nuetral dry yeast like US-05 and see what you get.