I suppose a Stanley knife would you the job actually wouldn't it! If it's sharp enough for carpets then it will cut through plant tissue cleanly I bet!
While we are on the subject, rooting penstemons in water has been 100% so thanks for the tip.
Im quite surprised actually at what people use to take cuttings because I thought most keen gardeners would want a proper pruning knife. So I think I may look at some of your suggestions instead like the disposable craft knifes