a few forum members asked for an update on this.
Well despite numerous explainations of the problem to BMW the rheostat control was replaced (a minor cost item apparantly carried out under the final days of the cars warranty). not sure if this has cured it but waiting for another day when the temp falls below freezing point (becoming less likely now but you never know with this british weather).
i was told if the rheostat fails then the ability to regulate the temperature is not possible. However this type of failure usually has the affect of freezing you to death rather than being cooked to a turn. The jury is still out on this matter
Hopefully the next switch to flick will be the air conditioning (he said hopefully)
ian s