What to use as a screed edge along a brick wall?
The conventional way to handle this problem is to use a length of 1.5 or 2 inch diameter pipe as the screed reference level. This pipe sits on top of a series of stakes that you place in the ground in a line a few inches from the brick wall. The stakes are pounded in so that their tops are the pipe diameter below the intended concrete height. Two nails in the top of the stake are set in on the sides to keep the pipe from rolling off the stakes.

The pipe does not need to be the full length of your slab. Instead as you pour the concrete from one end you screed along and then can pull the pipe back to the next stake. After you pull the pipe you toss some additional concrete into the gap where the pipe was and float trowel that flat with the rest of the surface.

The small stakes and nail tops just stay in the base area of the concrete slab. 

The idea to place the pipe say 6 inches away from the wall allows for easy back and forth working of the screed. (If you just had a thin board along the wall it would be very difficult to keep the screed on the top of that narrow board and there is no opportunity to use a sawing action of the screed which is necessary to work the pebbles in the concrete down from the surface).