White Spirit is the only method that actually turns the glue to a water like substance without affecting or having the potential to affect the paint. Heat gun works but it only softens the paint and turns it to a syrup like substance and as Boots says, overheat and you melt the paint too.
White Spirit is slower to do but safer.
Way to do it is to leave the PF in the machine while on its legs.. it has a natural slope down this way. Lift the very top edge of the Mylar very slightly and with a rag or syringe apply a little White Spirit to the join of Mylar and PF.. wait about 30 secs and youll be able to lift the edge a little further very very easily.... carry on down the PF, as you go further, the area you are applying White Spirit to  gets bigger so the process becomes faster as you move down.
Don't let the Mylar flop back down onto the PF as once the WS evaporates, the glue becomes glue again