Evening all. First, a big thank you to you all for your comments and advice, it has all been really helpful.
I went to visit the job this afternoon. When I arrived, some of the lights (downstairs circuit) weren't working at all. Tried a few switches, then some lights started flickering, then came on full. Opened up accessories and a number of conductors had broken at their terminals, several being ones I had terminated myself last visit.
@Marvo , you were spot on, the aluminium conductors do not tolerate being over tightened at all.
As a temporary fix I have used Wago 773s to join the ally conductors together and 'pigtail' a length of copper to terminate in the screw terminals of the accessories. The ally wires went into the wagos easier than I expected, and continuity was apparently sound after the fix. I imagine if the Alu-plus paste was used, this could be a good fix if anyone else comes across this problem.
Before I left I quickly tested the other aluminium circuits. The cooker circuit (10mm ally) tested fine, but the ring final (4mm) had problems end-to-end; 1400ish ohms on neutral and OC on earth. I assume this has been a long term fault as the breaker has been down rated to 20A by the previous spark. I'll have to take a look it it next week.
Anyway, the good news is they want to go ahead with a rewire in February, so there will be plenty of salvaged cable for you
@Lucien Nunes .
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I measured the lighting conductor with a micrometer, it was indeed 1mm². A few photos:
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