Thanks for all the input, but this discussion thread is wandering.

I don't need suggestions for alternative circuits, e.g. the zener-SCR circuits. I've used them successfully but the tolerances make the trip point quite variable.

I know this circuit is slow, but it's to stop some circuitry from overheating. As long as it trips in under 10 seconds then it will do what I need.

I guess my problem is that I'm not particularly familiar with Triacs. I thought they were back to back SCRs, and if I inject enough gate current they will trip and latch.