This is a bit late given its now November, but someone else like me may look this up. Base part of actual kettle on the HD4601 slides straight downwards off the switch housing, and the switch housing itself clips onto the kettle body at the top, so push it upwards gently and detach. I found the switch connections were corroded, did temporary fix but will need new parts soonish.
I think my fix was very cost effective, I don't think you need to buy a new one! Cost nothing, kettle actually cost $90 Aus new 11 years ago, so about 40 quid. Excellent kettle and still going!
Neil Mc, Tassie, November 2008