XPS 2710 downgrade to windows 8 but no product key

Hello All, 

Unfortunately I am one of the many that have gone through the black screen saga with the 2710. I did not realise the issue was win 10, so fater getting it up and running the first time it went black screen on me again. After following the cmos reset procedure and changing the jumpers around have managed to get it back up and running but it is on win 10, Bios i am on is A14.

Now to avoid it crashing again, I wanted to get windows 8 back on there. And was silly enough to think it would be straight forward.So I still have the original CD I got with the machine. popped it in and lo and behold it needs a product key, which they claim should be on the back of the PC, and although there is a number it doesnt work, the product key screen is asking for 25 digits, the back of my PC only shows 20. 

Any advice on what I can do? has anyone had success with anything else?

Many thanks

Shehzad