I have been using some 2k upol raptor truck liner paint, great stuff. However I have a full bottle of paint with no hardner - seems like I left the lid off and it's gone off!

It's expensive stuff and it seems a shame to chuck it away - especially as I need it. You can't some to buy the hardner separate so do any of the paint experts know if any 2k hardner will work with it?

I would try.
As far as I know, on some paints, if you add a tiny bit of catalyzer, you speed up the drying process.
It is not as quick as catalyzed paint, but they will cure at a faster rate.
HTH.