wiring a two way hall switch?

Two wall switch circuit involving downstairs and upsatirs lights in hallway.

Attempting to fit a new downstairs hall light.

Downstairs wall switch has two red wires - two black and one yellow earth wire.

Ceiling fitting a metal box in concrete ceiling with 4 wires appearing - two red and two black (no earth wire visible)

New light fitting/luminaire (bought Focus DIY store) states needs earthing.

Fitted two red to L live and two black to N neutral box in new fitting. Light works after circuit breaker restores power but either wall switch then trips CB to off when pressed.

Tried to earth with wire to metal box from fitting earth but no difference.

any ideas

Kind regards

Kim