Thanks for the tip and I did see that in some of the googled solutions but it is more the disk thrashing than the CPU usage that was preventing normal operation.
However I have been able to solve the issue by the following:-
Create a new folder xxxxPart1 for the project and cut and paste just the MP4 files used in the current episode of the project, rather than all the files recorded on the holiday. Delete any created HDP's from the original storage folder.
Create a new folder for all the remaining MP4s and give that a new name. From there I will cut and paste just the files needed for the next episode to a new created folder ie xxxxPart2.
Open the episode in 1. and relink the xxxx.MP4 cannot be found to the xxxxPart1 folder.
All is normal again but not sure why this issue occurs.
PS. Will respond to your PM with some thoughts on the Sony AX53 shortly.
Just an oddity with the method I used above is that anything I had stabilised with Mercalli V4 prior to moving the files and relinking needed to be re analysed after coming up with the banner across the clip which said "Analysis required Clip exchanged". The clip had been copied and pasted only....but who knows how Mercalli keeps track!