Cracked plaster in front of chimney

Hi all,
I have a 1940's brick veneer house with a gas space heater with a chimney. Above the picture rail in front of the chimney there is a crack in the plaster about 18 inches long. I have patched and repaired this crack 3 times in the last 10 years, but every winter when the heater is on the crack re-appears and I get some bubbling of the paintwork around that area. Obviously the heat is causing this to happen, but strangely it only is affecting the top area near the cieling. I have noticed that the plaster in that section is very thin in some places and am now at a point where I would like to replace it with a new piece of plasterboard. Is there anything I need to put in front of the chimney after I remove the old plaster and before I add the new plaster to reduce the heat eminating from there? (I'm thinking of something like aluminium paper insulation).
Also any tips on how best to do this job would be appreciated.
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