Maybe Jeremy couldn't quite believe he was dead.
Well,Bamber went to quite a bit of time and trouble to move Nevill and balance him on the back of a chair to check Steve.
Many also believe Nevill could not maintain that position without rigor mortis being well established.
Besides,if the cooker made the burns,Bamber didnt move him to get access to his back,did he?
No,and if he was simply unsure if he wasn't quite dead ,another bullet to the head where he lay would have sufficed surely.
I think you know Steve that if the Aga caused the burns then the police have some awkward questions to answer,as an intelligent human being you know this.