Ok, furthering this on a bit; adding some sockets into a ring so carried out testing around the ring to check for spurs, bridges etc. Cheap accessories from 1980s rewire. Readings are still significantly different even after lots of "plug thrusting" (that's my term for a plug giving it big time to a socket, you get my drift). For example:
Before plug thrusting:
Left: 1.4ohm Right: 0.72ohm
After plug thrusting:
Left: 0.97ohm Right: 0.69ohm
If I'm on the lookout for bridges on the ring, I'd expect <0.05ohm difference between different socket outlets on a L and N cross connect test, but with these sorts of magnitudes of differences caused (apparently) by oxidation, this test doesn't have much value.
In this scenario, should it be deemed that the accessory has failed and should be replaced to allow decent readings to be taken? Or any challenges on the 0.05ohm threshold? Thanks folks.