Its an issue with the Latest update of Denim, My 830 would not allow me to install 10, i had to follow what I've written here to make it work.
What you need to do is restore to the earliest version of Denim via the recovery tool.
power the phone up, ignore all the setup process and firstly go into updates and untick the auto update box.
Then connect via wifi and install the insider app, check for updates and it will allow you to chose fast or slow.
it took me about an hour in total, as it downloads 8.1 update 1 then you need to check for updates again to get 10, but for some reason the version of denim we got updated to is not compatible.