these photos show the process. Straight finished plate, bent starting plate, then process of ironing and the two bent plates I started with.
Well I have the answer for you which I have used with all the plates I've owned the old and new ones.
What you need is a hot iron, ironing board, cotton tea towel and pair of oven gloves. Put bent plate on ironing board and cover with tea towel. Iron backwards and forwards for several minutes and then using the oven gloves press firmly on top. Turn the plate every few minutes. This process will take 5/10 minutes per plate but straightens them perfectly.