Main reason could be matt emulsion on silk (?) combined with heat from rad. Did it crack on drying or when heat applied..
Is the wall greasy from washing being hung on rad and touching wall.. or erm cat box on rad etc lol.
Abrade with sandpaper/ 120 grit, wash if it is dirty, then try recoating..
just the damaged area, if ok recoat all if ithe repair don't blend in.
Depends on paint too..
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