[kinetic (Steve)]

Ok here is a rough circuit.
Hopefully no errors.
I'm not sure what the D1/D2 small signal diode setup does
on the 555.
I'd be checking the output Pin 3 of the 555 and turning the
potentiometer to see if the PWM or average voltage out
changes. A digitaml multimeter might not show this, I'm
not sure what the 555 is oscillating at. If it is up in the KHz
the reading probably won't make sense on a DMM.
An analogue multimeter might show variation, an oscilloscope,
definitely.

Edit: oops I forgot VR1...it's across the heater outputs exactly as
the C2 is shown. (varistor?)
Clearer circuit added with Varistor and LED

Steve