Sudden leak, I assumed old o-rings were bad. I put new o-rings. It leaked straight away! I then covered the pipes in a large amount of silicone. After tightening down, it was perfectly fine and not leaking anymore.

I feel there may be a crack in heater core entry area, which the silicone has covered. There is no point
in replacing heater core, since this has fixed the issue and will probably last a decade or even more with this solution.