I believe a 60 min FP emergency stairwell should be kept sterile. However at a 3 storey office there are 3 electric wall heaters. One has stopped working and I have been asked to get it mended. I have said they should be removed completely. But because it's cold they want heaters on.

Am I correct in refusing this request?

You are quite correct and an emergency stairwell should be kept sterile, but that is not always possible and you should reduce the fire risk to as low as possible. Could they be replaced with a more safer type of heater, is there any flammable materials close that the heaters could ignite and could they be interfered with by passer by.

As I have said if they cannot be removed make them a safe as possible also check out the Fire Risk Assessment maybe they have been considered a significant finding.