Working with CONDCHECK I got confused. The condition check works to mark a field that is filled out incorrectly, but the condition is to be read as “these conditions should be satisfied, if not…”.
Which makes sense; you write the condition that you want satisfied. If it isn’t you get the message.
My problem was that I thought of it in the context of if … then; if this condition is satisfied, I should get the message. Which I now realise was wrong.