Many thanks for the suggestions Colin and Malcolm. I got one from the Distributor Doctor and have been promised also a spare older one. Problem solved.
David. A word of warning. Lucas made two versions of the N type rotor arm, which to most people seem to be identical - but they are not. The blade on one version is very slightly longer than on the other and needs a small amount of trimming if it is not to destroy the new rotor and/or the distributor cap. Carefully test fit it before using it.
Many thanks for the warnings. The Distributor Doctor describes both types and the reasons for them on his website. Mine is a distributor cap where the wires exit horizontally.