You won't get the lock pick until you have the mask. You need to keep exploring until you find a lord with a sword, he has a key on his belt which opens the way to get the mask. This is all past the garden.
As I said, there is a lot of running around to do, lots of stairs and corridors.
There is a key that opens the sewer door in the garden which leads to the area you need. I can't remember where the key is though.
I will have a look 'round again to get a better image of where