Further update: After posting I checked 'AI' Claude in this case for Rover V8 tappet noise, it suggested that after running with no oil or standing idle for a period of time air in the 'lifters' could take a few minutes to be fully removed. So, just ran the engine again until hot and the tappet noise slowly disappeared! Phew! The oil pressure at tick over is just under 2 bar and around 3 bar at 2000 rpm. Not taken it for a run yet, that's tomorrows adventure!