Sorry I was working from a later revision of the circuit, with more component values on it, when I asked that question.
I'm wondering now what is the function of the capacitor C4? Is it smoothing the inputs to U1B?
And could it be possble that the SCR is, in fact, a triac? I ask because the part number is Z0405MF. I think I understand why it shouldn't be - because if it were then a positive OR negative current at its gate would trigger it whereas a SCR would only be triggered by a positive output from U1B?
And what is the point of using a DIAC in this circuit?