Ok thanks for the reply.
There is no setting in the BIOS to affect the fans. Dell BIOS's are notoriously light on features and there is absolutely no way on a Poweredge 600sc to set it so the sensor is ignored
But I take the point about the fans its just I have already spent £40 on a huge zalman heatsink and a quiet cpu fan and also £7 on a quiet psu fan - I am loathe to spend much more on other fans if I can help it.