I was given a 5L pot of Laura Ashley (no less) paint that's been opened
but almost full.

The can says 'use within 6 months of purchase' and indeed google
searches tend to repeat this. The paint is about 4 years old - as long
as it's taken me to get around to think about using it ;-)

What's the worst that can happen? I don't understand how paint can
degrade. Should I just chuck it and buy some more?

Thanks, Rob