Left yourself a bit of a problem there by the looks. There are the obvious solutions of using stainless or plastic angle to hide it but the success will depend on the quality of materials and fitting and even the decor of the room.
If it was my problem I would be cutting back that plaster board at least an inch or even to the start of the brickwork, putting on a plasterer's angle bead and returning the plaster to the brickwork. In my opinion it would look much neater more finished than using some trim. More work for the plasterer so don't rely on his support for the idea. :) Can't find a photo to explain but it's effectively making a small reveal to the brickwork.
Personally i would get some 50mm angle iron & cut the top left & right corners at 45deg & fit with a heatproof mastic. If you have someone local with a welding plant, weld the 45d corners & grind flat, then paint black. A welder/metal worker will take about 1/2 hour to do the whole thing.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!