I am quite sure that I got the wiring straight thro.
I metered 1 for 1 from the cable right to the RJ11 plug.
What is more, I made a direct replacement of the Master outlet for an Extension outlet which has no suppression and all was well again.
I know this is puzzling and I am not inexperienced in electronics and am convinced that I got the wiring correct.
It would be very interesting to know if anyone else has tried successful surge supression on a La Crosse unit with its strange technology.