Hi Martin and welcome to TM I don't know what would cause that but to help you find it is a link to the gomog site with the Plus 8 wiring diagrams. The impulse feed is White with a black tracer on the cable well worth checking these connections end to end I would have thought.
Plus 8 Electrical Someone will have seen it before and hopefully come along soon.
p.s. You are better of in future posting technical stuff in the Servicing and Maintenance section as the Malvern Arms is for very general stuff and can be ignored quite often so your query may get missed.
