Finally after about 10 goes I'm on W10 on the Linx, only another 2 systems at home to do.
What worked in the end in case someone else experiences this is to do the following;
Copy any files you want to keep to another drive other than the system drive.
Delete some of the Metro Apps that you don't need, the store will remember which ones you had so you can reinstall them again later if needed.
Make a note of your Wifi access password.
Use the PC Settings menu from the charms bar and select the option to reset the system.
When it restarts and goes through the out of the box set up routine, I chose custom settings and switched off Windows Update (Don't need to bring W8.1 up to date first).
Connect to your Wifi and it will give you the option to sign in with your Microsoft account and the option to restore settings from a previous cloud saving of your settings to MS if you had it configured that way.
When your back in to W8.1 it's quite lean, about 20GB free space, the Metro interface has placeholders for any apps that were deleted earlier as a way to choose which ones you wish to re-download instead of having to go back into the MS Store and go into Accounts / My Apps. Of course at this stage it's not necessary to reinstall Metro Apps.
Grab the Media Creation Tool from MS, run that and it should work out OK to W10.
Now in W10 I go into properties on the System Drive and do Disk Clean Up and Clean Up System Files, go through the list and tick delete next to Windows Upgrade Files and Previous OS Install, providing you don't want to go back to W8.1 through the OS, I presume you can still go back from the recovery image drive though providing MS didn't change the program in W10 to not recognise Images from previous OS versions.
Now got 17.5 GB free, 2.5 GB less than I had with the W8.1 reset, but not bad.
