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I guess it is tripping the RCD rather than the MCB so the Granny with car connected is causing an earth leak big enough to cause a trip.

Several possible causes:

1) There is already a lot of leakage from fish tanks, ovens, USB chargers, etc. that a teeny bit more is pushing the RCD over the 30mA limit.

2) The MCB is overly sensitive.

3) There is a fault in the granny/car that is causing a legitimate trip and the other RCDs are faulty.

This is an inherent problem in having loads of circuits on the same RCD. Many years ago RCBOs (breakers with both overcurrent and earth leakage protection built into a single device) were too expensive so either the whole house was put on one or the house was split int two with half the circuits on one RCD and half on the other. Nowadays RCBOs are cheap enough that there is only need for separate RCDs if you are exercising extraordinary economic restraint.
 
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