The Windows 7 system was activated using SLP (OEM BIOS Activation).
First issue with taking advantage of the free upgrade to Windows 10 offer - Automatic Update was turned off on my PC and I didn't have the Get Windows 10 app installed. Nice to know that this isn't actually required - just download the MediaCreationTool instead.
Now after some reading it appears that in order to have an activated and licensed copy of Windows 10 an inplace upgrade must be completed first - using the MediaCreationTool and selecting the Upgrade this PC now option. I won't bore you with all the details - after running through a range of self explanatory steps I installed Windows 10 Professional (32-bit) without any issues and it activated as soon as I connected to the internet.
During the above process a hardware hash is generated - this is apparently stored on a Microsoft Server. This hash is unique to the machine the upgrade process was completed on.
I'm not a fan of installing Windows by upgrading in this way - so on to a clean install. I used the MediaCreationTool again and this time selected the Create installation media for another PC option > "ISO file" > and set "Architecture" as "Both".
I then rebooted my PC > formatted the system drive (diskpart) > ran setup.exe from the .iso file I'd created and completed a fresh install of Windows - on connecting to the internet Windows 10 automatically activated - thanks to the hardware hash stored on the server following my inplace upgrade.
Now here's the fun part - I upgraded from a Windows 7 32-bit system. Just for fun I though I'd try installing Windows 10 64-bit from the .iso file. I booted to WinPE and formatted the system drive (again!). Booted the .iso created earlier and choose the Windows 10 Setup (64-bit) option, then completed the Windows installation.
On completing the installation I connected to the internet - the 64-bit Windows 10 also activated from the hardware hash generated following the Windows 7 32-bit inplace upgrade!!!!!
This was a test. I now plan to revert back to Windows 7 or possible 32-bit Windows 10 as in my opinion running a 64-bit OS on my hardware is a pain in the arse and a waste of time.
The .iso created by the MediaCreationTool is interesting. It contains both 32-bit and 64-bit WinPE with menu options in the BCD store. Contents of the BCD store from my Windows 10 .iso -
The following site was very useful - http://winsupersite.com/windows-10/ (particularly http://winsupersite....ws-10-activated)