I asked Focusrite and this is their response:
"The Saffire Pro 26 i/o has been discontinued for some time and as such development support for it has ended, so we will not be releasing a driver update or supporting the interface on Windows 10. There is a chance that the interface will work anyway, but we have not tested this. One of the main issue you will face however is the lack of the legacy firewire driver in Windows 10. The Saffire requires this driver to work, but Microsoft no longer include it in the OS. They did release a standalone installer for this on Windows 8, so there is a possibility that they will also do this for Windows 10. If and when this is available you may then have a chance of the interface working with the old driver. "
So no real surprise there and I rather fancy spending some (more) money.
Besides, I'm not getting super low latencies with the 26IO so look out wallet!