Right, scratching my head a bit now, I rebuilt the circuit according to John's diagram (minus the reversed diode behind the Zener, which isn't on my board); I get 0v & a hot resistor...
This is on my test bench, I am feeding it anything from 0-35v, 0-2A...
If I give it less than 24v, the resistor stays cool. Above 24v, it gets hot.
This _has_ to be nearly what I had before....
I have a spare N4002(ish) if adding the reverse diode will make it work...