In the meantime, I appear to have "won" an Isotek power cable worth £60.
They want the distribution block back, but they've said I can hang on to
the power cable for now. Might try opening up the plug and see exactly
what type of filter they've put in...


Yes, see what's adding in extra resistance and or inductance.

Don't fancy doing it all again with a storage scope across the mains to
see what's happening to the waveform and voltage?, to see if the
filters/power cords etc are seriously affecting the mains power
increasing the source impedance etc, into the units under test?........

--
Tony Sayer