As far as I know there’s no separate earth connection in Rega decks and it is combined with one of the audio negative connections (left channel negative maybe). I’d try connecting a fine wire to the earth post on the rear of the amp and carefully touching it to the metal cover on the underside of the cartridge whilst playing an album but be very gentle and use an old LP! If there’s no change then earthing may be ok. Or check for continuity with a multimeter from the cartridge metal cover to the earth connection on the amp with the phono leads connected and amp powered off.