Cheers for the advice Gerry, the bit was definitely 'screaming'... I was feeding it slowly because of the DOC being 4.2mm for a 4mm sheet and me worrying about snapping my new bit due to its length.

Can you tell me if what I've read about the DOC being half the diameter of the bit is true? I've had success with cheap Chinese carbide up cutting bits, cutting deeper than their diameter and everything seemed to be coping.