Ian Stirling wrote:

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

Wow, that's high -- overhead or underground distribution? Have you
tried complaining about undervoltage?

If you've got a voltmeter and enough clue to connect it to the mains without
killing yourself, this is easy to measure.
[...]
Meausuring and using a load on the same circuit isn't a good idea as this
adds that resistance in.

It's absolutely essential to measure on a different circuit to the one
you're loading - or straight on the CU busbars, using fused test leads
of course - otherwise the results you get are meaningless.

--
Andy