Okay don't know if this will help all but after two weeks of trying different things I got it working (no thanks to sky support who were less than useless)
So I started in Safe boot mode (hit restart, hold down SHIFT key and let go once you see the Apple Icon) You will see "Safe boot" in red in top right of the screen as it starts up.
Go into Applications folder in fiinder and delete ANYTHING you feel might have anything to do with capturing video or screenshots - I deleted, for instance, Hide My @rse, Real Player, Join Me video conferencing, 5k Video player, a couple of remote desktop apps, Flip Video etc. Basical,ly anything that might cause a problem. My thinking, if it works I can always reinstall them one by one to see if it breaks Sky (which I use every day). One thing I did realise is I had a lot of crap I hardly or never use anyway, so good clean up exercise.
Then go to Sky go website and download and install the new Sky Go App. Open it, run a catch up programe (video will be choppy or just be a still as a lot of things that need to help video probably didn't load in safe mode) but you should NOT get the error 9903 and it is actually working you just can't see the video. Maybe move the programme to halfway through so you know it's actually running. Again you won't see much but the slider will show the programme is halfway through.
Now reboot the machine (probably opens a lot quicker as all the rubbish that was opening before is now deleted:-)
Go into downloads and move your Sky app into the applications folder (if it isn't there already, not always is for some reason?)
Open Applications - Start Sky Go - Start a catch up programme (probably asked to sign in) and Bingo it works (well at least it did for me! Good Luck.
Sorry this was so long, wanted to be clear. Please post if it works for you (following every step in order) and also if i didn't work.
To be honest I don't know if it was booting and installing Sky Go App in Safe Boot or all the crap I removed or a combination of both. But I think doing it ALL is good for your Mac clean up anyway, and the whole process took 20 mins or so. so give it a go!
Cheers John.