Jon - Crydom offers several series of optically isolated solid state relays capable of supporting load voltages of 240 VAC and above. Current ratings are in excess of 10 amps. Turning the relay on and off is done using a logic high or low capable of sourcing 10 mA or so - I've used 3 mA without any difficulties.

Using one of these relays allows you to apply power to the load without keeping track of the power line cycle to trigger a triac. All you need to to do is maintain a PWM frequency that is (much) longer than the power line frequency. How much longer depends on the size of the metal block and the thermal time constant between the heater and the sensor.

Bruce