I really dont recommend the heat method as it will soften the paint below also.
The only method i use these days is White Spirit.. its slow but fautless so long as you dont rush it.
Leave the PF in place as it has a downward slope already.. starting at the top carefully lift the edge of the Mylar slightly. Use a rag or even a syringe and apply White Spirit to the join between PF and Mylar, Within seconds, the mylar will just lift off. The downward slope ensures the WS continues down the PF as you go and so slowly she just slides off.
Method works because WS is a brilliant glue dissolver allowing the Mylar to just slide away.. just keep applying WS as you go.
It doesnt soften paint and doesnt react with dried paint.. so long as you allow the WS to dissolve the glue there is no tension as the Mylar lifts away... even the most fragile of PF paints can be saved successfully