I'm old enough we had them at college years ago !
My apology if you thought that was meant for you,it was just old folks banter
Too many problems listed to comment on them all
Next issue is on downstairs sockets, tests r1,rn, fine but r2 relationship 2.5/1.5 ratio is way off ! r1 0.44, rn 0.44, r2 16.8 kOhms !! wtf ?
Thinking nasty connection on cpc somewhere anyone seen similar before ripping apart the circuit.
16,800 ohms will likely be open circuit rather than a nasty connection,that reading should be assessed in conjunction with the insulation resistance test on that circuit,it could be a reading of the cpc s of a open circuit with continuity via any appliances and associated copper pipes