Hey mate - yeah not coming from the hose connection area (I'd already replaced that o-ring at some point anyway, probably when I changed out the thermostat or water pump).

It's somewhere on that RH tank, I suspect where it joins the aluminium core. It's more or less as fully investigated as it can be besides pulling the radiator off, and if I go that far I might as well replace it.

Haha, I do frequent the car wreckers here often (self service, pull it off cars yourself). I'd normally just go with Mahle, but I think I read somewhere NRF made the original radiators for the E46, so can't be too bad.