Not being the patient sort I've gone and ordered an Electrex unit before awaiting advice. The Newtronic one is about the same price (£70 ish) but is only rated at 35A.
Northwest Motorcycle Services do one which looks really robust, rated at 50A but is more expensive at £80 (+p&p?)
I see that Electrex also do replacement CDI units for Ducatis which claim to be programmable, no technical info though.