I had a case last year where I replaced a burnt-out pull cord isolator like this one. After removing the "worked" (brittle) bits of damaged cable and made good terminations on a new switch, I did an R1+R2 at the shower to confirm everything was spot on, and I got poor (high) readings.
It turned out there was another 45A DP isolator, a plate switch, downstairs in the service cupboard near the consumer unit. This had also been poorly terminated and was burnt out, and also needed replacing.
It reminded me of the importance of testing, even for a relatively simple "straight swap" maintenance job.