How can I move my Apache files to run off the imernal SSD please? Is this possible?
which files you are looking for? Web server root /var/www or some application files?
Web server root /var/www
well you could transfer entire rootFS to the HDD but this require an empty disk.
How do I do that please?
Using drive manager
Currently got dietpi userdate on the HDD will transfer rootfs muck anything up?
Transferring RootFS will format your HDD. Means, you would need to transfer user data directory back to SD card first. But not sure if this is possible as user data might be bigger than your SD card. But if size fits, copy everything like user data back and transfer whole rootFS afterwards. This will include user data at the end as well.
Back to ext4 on SD card?
yes, but you would need to check if the amount of data fits before hand.
Thank you for your help. I think it worked 
Looks like root partition is located on sda1 now. It should be correct if all apps working fine.