These are strong: http://www.screwfix.com/p/fischer-hm-plasterboard-anchors-m4-x-46mm-pack-of-20/38777
How they work: http://www.free-instruction-manuals.com/pdf/p4233075.pdf
Find out the diameter of the bracket holes to choose the correct size.
Sorry, I wrote something which I then deleted. I should have use the word "deleted" But now I'm thinking that my original deleted advice might be useful after all. Sorry for the confusion. I was in a rush!
Try these: http://www.screwfix.com/c/screws-nails-fixings/spring-toggle/cat840118...
Is that an electric fire? The reason I ask is that you might have a blocked flue without even knowing it, therefore no ventilation. Rain might be coming down the chimney and it has no-where to go. Does the chiney breast have air vents? Is the chimney capped?
jimmies78, my chimney breast is the same width as yours. I recentlly opened it up and removed net 90 bricks and installed five new lintels. I didn't use a prop! My house was built in 1987, yours looks older. Pictures...