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The unified meter control is in the instrument cluster. From memory on the series 4 there are only 2 screws in the dash facia around the instrument cluster. Drop the steering column, remove the two screws and then gentle pull the facia towards you. The bottom of the facia is held in by clips that can be stubborn to pop out.
I still don’t think your problem is within the instrument cluster. The symptoms don’t indicated this to me.
Do you have a HF radio or any other EMF producing device fitted to the vehicle?