lawman Is this a common pixel fold issue
No. Phone should never overheat, and should fully charge in 1-4 hours regardless of model and charging mode.
lawman how can I evidence this to get a refund/return from cex who will look for excuses to say its not an issue?
By having tried to resolve the issue yourself first. Start by doing the steps I suggested in my previous post, to see if it is a specific app you have installed that is consuming a lot of battery. Try factory resetting the device, and see if issues happens even if you haven't installed any apps. Try going back to stock OS. Rule out all causes that can be you as the user. If issue still happens, overheating and not being able to charge overnight is definitely strong indications of hardware fault.
If the device is still in warranty, the reseller might send the device for repair, or replace it.
lawman I'm concerned if this is a GOS issue with pixel fold and if I should remove GOS before returning to CEX.
I would reflash stock OS and confirm the issue happens there too before return the device anyway.