Discovery V8 Heater/Fan

Have a G reg Discovery V8 (with air/con).

And the heater has stopped working. on the 2 normal fan settings the fans do not work at all.
But as soon as you switch the air con on it works! (That in not very helpful in this wether!)

Have checked all the fuses but not sure waht to try next!

Please Help!!

It's a funny one, i have looked at the wiring diagram in Haynes book so it's not to clear but is seems that the fan relay 'may' be the problem at that is the first point after the fuse. The relay is located behind the kick panel drivers side, thats also taken from the book so which one it is when u take the panel off I don't know, but just listen for the click.

Have spent all afternoon trying different things, even went to scrapyard and got relays to replace them all! (still not sure what relay is for the heaters). But still no go!

Is there a heater resistor on discos? (i'm guessing yes).normally when the heater resistor fails the blowers only work on full speed as thats a direct feed from the switch.aircon when swiched on will put the blower motor on a slow speed and as said previously this is controled via its own relay and not the resistor.