All done! It all went smoothly but I did not use the Synology tutorial linked above, here's what I ended up doing;
1. Created a new fresh USB using Win32DiskImager and XPEnoboot_DS3615xs_5.2-5644.5.img (my N54L was already up and running using this img).
2. Transferred all the hard drives from the N54L to the Gen8
3. Connected the Gen8 to a monitor, booted it (WITHOUT the USB) and in BIOS set boot to USB and set ACHI. Saved all setting to default.
4. Switched the Gen8 off, inserted the USB and then booted the Gen8, waited for Xpenology to boot and then selected Install/Upgrade when the selection screen appeared.
5. Let the USB install
6. Once installation is complete (or as complete as it gets, it didn't seem to take much time) I used the IP address of the DSM page in my browser (in my case it was http://192.168.1.xxx:5000/webman/index.cgi) and to access and enter the DSM web screen as normal.
Other than the above, I didn't have to do anything else, the DSM works as it did on the N54L, no data loss, didn't have to change anything within DSM and didn't have to restore my configuration. This makes sense as all the DSM information was already on the hard drives so nothing really needed to get changed. I suspect I could have used the old USB Xpenology boo tUSB but thought a fresh one would ensure loading of the correct drivers etc.
Anyway, everything is working and my thanks go to abced and of course to the Xpenology team for a great piece of software!