Iain, I'm another having serious problems . I have 3.12 installed and can't get 3.13 to work as well. I can't get the full version via lostkeys because I have no idea when I purchased original 2.1 or 2.2 but certainly at least two ISPs ago and 5 or more years - emails gone. I have merrily upgraded 2.3, 2.4, 2.5 3, 3.1 etc, paying when needed with no problems. I also had RAF (obviously no longer, but still needs ticking?).
I have at least 5 upgrade keycodes. Control panel programs says I have 3.1 in FSX installed 05/10/2015 (after the 3.12 update). I even did FSX then P3D then FSX again to get registry right . . I have no uninstall entry in the registry though do have uninstall.exe in Scotflight folder with shortcut in start menu and it would work from Control panel as well
it's head-banging time . .
I have since tried complete uninstall, reinstall 3.1 version - successful, re-install 3.12 patch - successful, but trying 3.13 still gives no version found for this simulator in registry . . . yet it's there!
I should add that I use what I have labelled as 2.5 upgrade installer keycode successfully - so not lost key - where do I get the full 3.13 installer?
Mike