It could be a common earth issue Mark, but the one for gauges is shared by six other black chassis earth leads, and you only have the one fault area. I would be looking initially at the VDO master gauge connector behind dash. This is a long 26 way plug and socket although carrying only eight wires, fed by a 500mm long sub loom breaking out from main chassis loom. The black wire within is the common earth. Maybe break and remake connector as a start.
