I have used both isolation and auto transformers for tunnel boring machines and tower cranes 11kv to 10kv and 240v to 220v but this is expensive and I would first try DNO then if that fails use HF florescent lighting many of the compacts are HF and can take variable voltage better. But not any inductive florescent units.
I am going to try this again with her. The problem is she has these fancy light fittings that are all verdigris (green metal to me) and they do not look good with CFL's. But it will be her choice.
When I came across the same problem a while ago I contacted the DNO who came out and attached voltage recorders to their cutout. They then came back after a week and took the recorders away. After they looked at the data they then came back and, as elfist2 says, altered the transformer.
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