Thanks for your replies Steve and David.
Steve: the control you describe is absolutely correct for what I have in mind.
The controller will control a 240VAC resistive heating element, an electric oven element to be exact. 2400Watts maximum power.
What I vaguely had in mind was an IC555 using the duty cycle output pin to drive a Triac. The 555 lengthens or shortens the length of the 'on' period in the duty cycle controlled from one of the voltage input pins. This input might come from an op amp (if necessary) or straight from the voltage drop across some kind of temperature responsive resistor device.
What I would like is a simple device that runs at 240VAC so I don't have to build in a complex low voltage DC regulated power supply.
I would also like it variable in the temperature that it controls.
I would also like it to be 'universal' in that I could use the same circuit to drive heating elements of say, 50Watts to 2000Watts. I could use it for controlling a drying/varnish baking oven, water jacket temperature for a PCB etching tank or even my offive laminator (which is out of action due to its temperature controller being US.
Any imformation would help.
David: I don't quite understand what you are getting at, what is a PT100 or a PWM? I know what a Wheatstone bridge is though.