That probably isn't an RCD.I've disconnected the workshop and just connected the flat to the double pole rcd (really old),
If the supplier is refusing to install a meter / connect the supply, it's because the installation is dangerous and must not be used.supplier (seaboard) want to put new meter in but wouldn't touch it with earthing in currant state,
In connecting that to the supply you have put yourself and others in danger. People could be killed as a result.