I hope that the advice to stir it before distilling a smaller amount into the paint pot has reached everyone.
Zinnser 123+ does not hold the solids in solution for very long so you need to stir it regularly otherwise you get very heavy going at the bottom of the pot.
Thanks for the tip. Will be doing the ceiling in the en suite this weekend with a bit of luck. Will try to remember to stir both frequently and vigorously.
Do you have a toolstation or screwfix near you? They stock zinsser - since @Airhead recommended it, it's all I've used