If the DNO change it, what would they replace it with - a 100mA RCD? (which obviously would not provide adequate shock potection to the OP's installation). Given what we've been told about the load side of the VOELCB being split to supply two CUs, might the DNO be persuaded to replace the VOELCB with two 30mA RCDs for him, one to feed each CU?From what the Op says this device could well be the DNOs, so what kind of electrician would change it anyway, possibly breaking seals to do so? Op your first port of call must be the DNO to change it, it should not cost you anything!
Kind Regards, John