So having cleaned my PC run all the updates, made a back up, tracked down the product key (Thanks jih64 and others) and set to upgrade, I face the error message:
 
Compatibility Issue:
Intel Graphice Media Accelerator
3600 Series
The display manufacturer hasn't made you display compatable with windows 10. Check with you manufacturer for support.
 
 
So is this upgrade:
a) dead in the water?
b) too early, as it's only just come out?
c) impossible to say just yet?

Apparently, the GMA 3600 won't support Win 10 and Intel is "working on this". See here: https://communities.intel.com/thread/77701  

Try this anyway, you might find other drivers of importance:
https://downloadcenter.intel.com/download/24345/Intel-Driver-Update-Utility
 

Ah this isn't the end of the world, it's 200 quid laptop coming up to it's 3rd birthday. I had just spent a lot of time preparing it  for a clean slate. Being I have 3 laptops to run through the process, made sense to try this one first. Also this would benefit from a sleeker OS, being it's not so powerful.

I'm going to say the upgrade is dead in the water:
 
http://www.intel.com/support/graphics/intelgma3600/sb/CS-034806.htm
 

Why has Intel not provided Windows® 10 drivers for Intel® GMA 36xx series?

Intel® Atom™ processor (formerly code named Cedar Trail) based entry notebook and entry desktop PCs were designed and validated to support Windows 7. Windows 10 is not supported on this platform but is supported on newer Intel platforms.

looks like all hope isn't lost: