Jimmy, your garage sounds HUGE !
So.. Stay away from DIY self levelling scree stuff. I did out outside loo, only tiny but it was a nightmare, the stuff didn't self-level itself at all and I ended up having to level it all by hand after it set.... Big garage would be a nightmare.
Get someone in to do it for you if you go down that route. I just painted my garage floor with proper floor paint and it's lasted well...
Alternatively, get Vicky to do it
