The circuit should now look more like this with the diodes added to a 10.0 volt zener and the capacitor added to eliminate spurious triggering.
Ah,.. just re-reading your comments again, that 5K isn’t doing much is it?! This is because the zener ‘can’ handle 1 Watt but in this case it’s nowhere near that, so the actual current flowing is very small. That means that where I have suggested a 5K preset, to get enough adjustment it maybe should be increased to as much as 50K or more, or it’s still going to be pretty ineffective.
I think maybe I should get to bed earlier so that my brain is actually awake when I post replies in here! Haha.