Hi, I got a responce from my Ultimaker support ticket.
The method that was recomended via the video here: Printhead PCB Installation (UM3/UM3X) - YouTube
This method was recomended along with a caviate that the print core should be heated prior to disasembling the print head assembly.
This sounds good, and I began the disasembly prior to switching off and unplugging the printer. I only had to disconnect the cable, undo the 2 rear screws which go through the full length of the assembly.
Byt the time I had removed the print corethe core had cooled, so removing the filament was difficult and additional heat was required. This solved the issue.
I then re-assembled the print head assembly, cleaned the cores, calibrated, performed maintenance and everything is now printing well again.
I hope this helps someone else if they have the same issue.