Ok, so some success !
Here's what I did.

Exited the hanging install process.
From windows 10: Uninstalled Antivirus. Disconnected internet. Clean boot.
Then inserted USB with Win 10 install...

Hi Joergi,
Yes SP1 is applied - according to control panel > system

Unfortunately Media Creation Tool is running on the machine now : Getting Updates - 'Checking for updates'
In absence of other...

Toshiba Satelite
Originally Vista, Upgraded to Win 7 Pro (32bit) SP1.
Celeron Dual Core T3000 @ 1.80GHz
4GB RAM

Background
A few weeks ago the laptop's power seemed to just 'die' when booting...