As Kme has asked, Where's that from ? The Main earth has to be not less than half the Neutral conductor size. As the On Site Guide ( yes, its only a guide) states a minimum of 25mm tails should be used, then the earth has to be 16mm and the bonding 10mm. These may be in place now. As the C/unit is being changed the latest version of the regs has to be upkept. This means Rcd's on most, if not all circuits and the main earthing and bonding.A 17th ed db should not need 10mm bonding so the original 6mm will be ok due to everything being on a 30mA RCD