Hi flatfoot,
This is the sort of thing your looking for but they're not cheap
http://uk.rs-online.com/web/c/power-supplies-transformers/transformers/variacs/
You will probably want to combine it with a normal single output transformer to drop from 240V down to 24V and then use the variac to sweep from 0 to 24V (0 to 100% supply)
Another way that may be cheaper -
If you can find a dimmer switch rated high enough
http://uk.rs-online.com/web/c/lighting/lighting-components-accessories/dimmer-switches/
Then you could use a fixed ratio transformer to match that to your desired output range
The variac is better generally as it will deliver a full sine wave phase to the load but for heating some resistance wire the switched output of the dimmer should be fine. Would have to experiment a bit.
P.S. The reason why the dimmer is less than ideal is that it needs to be supplied with 240Vac and if you were to use that directly then you would have a very small range of adjustment. Putting it's output across a 24V output transformer would give you the limited range you were after but the limiting transformer would be inefficient as it is being supplied with a pulsed 'duty cycle' wave.
Variacs are better as they can be supplied with any voltage ac (within their operational limits) and still be fully effecient.
Triac switching (dimmer) is much cheaper though for the same power.
P.P.S. Dont use a dimmer on its own! It's output is not isolated from the mains supply and the best part of mains voltage will be supplied to your burning wire with obvious safety concerns! You would have to follow the dimmer with an isolating transformer.
