With a body in a bent over position it would have been far harder to pull the jacket up from the waist because it would have tightened around the shoulder area. Had the burns been inflicted around the waist area, the act you describe would have made sense but you can test it out for yourself, can't you?
I just did.
I put myself in a similar position to Neville and then pulled my shirt up. It easily went up to the top of my shoulder blades. My shirt is certainly tighter than pyjamas would be. It would be even easier for another person to do it, such as Bamber on Neville.
I'm sorry but for me there was no difficulty in Bamber pulling up Neville's top and burning his back. Using the rifle and aga. Neville was in the perfect position and had the perfect top.
Dr Caruso, Bamber and the 2002 appeal agree. Although Bamber claims Sheila did this.