The Senator Diner http://dine.to/wap/profile.php?id=18624§ion=details would be my pick for a decent place close to the Cathedral.
How many people are in your group, and what kind of brunch would you like?
Blake House on Jarvis (pub brunch and a patio), Lola's Kitchen on Church (interesting brunch), Hot House (mediocre buffet brunch around $25/person), King Edward Hotel ($50+/person for a deluxe buffet), Origin (innovative brunch), Queen and Beaver (British pub brunch http://queenandbeaverpub.ca/ ) and Le Petit Dejeuner (tiny place serving Belgian breakfasts- stick with the waffles or benedicts, expect line-ups after 10 am, not recommended if there are more than 4 in your group, no air-conditioning) are within a 10 minute taxi ride or 20 minute walk of the Cathedral. If you have more than 4 in your group, might be best to go somewhere that takes reservations. Paganelli's, The Vic Public House (haven't been, so I'm not sure what their brunch is like) and Beerbistro are within a 20 minute walk of the Cathedral, and all 3 take brunch reservations through www.opentable.com .
Enjoy your visit to Toronto!