Russ introduced me to this stuff - 3M surface cleaner.
It's almost pure alcohol apparently but its wargames use is limited to being one of the best brush cleaning products around.
Put some on a cloth, wipe over the brush and - voila! Lovely clean brush with all paint removed.
I've managed to resurrect some old clogged brushes using this stuff - with the benefit of an overnight soak.
Not cheap (£8 each) and available only from trade suppliers but should last for ages.