Briggs 12hp Carb, removing main jet

Hi,
New to the forum, hoping for some expert help....I've posted in the parts forum as I am also looking for a carb, but here's a more pressing question:
 
I have a pretty old Lawnflite 548 ride on which has been excellent but recently developed rough running and a fuel leak from the carb.  I tried cleaning the carb, and a new float, no joy. I think the float is sealing OK, but fuel is still seeping through when the engine isn't running, perhaps via the main jet (which then opens the float valve)?
 
So I went to remove the main jet, and have hit a problem.
 
I cannot get the main jet out of the carb (by main jet I mean the one which the bowl holder screws into). I think it has been tried before as the brass is pretty messed up and it is hard to get a screwdriver to bite. I've tried penetrating oil and lots of heat, but no joy.
 
Does anyone have a recommendation on getting the jet out without trashing the carb?
 
And any thoughts on why fuel is seeping through?
 
The engine number is 280707-1028-01 and that gives a carb body part 490473 
 
Many Thanks