Rusted core plug in Traction Avant cylinder head

A Traction Avant came to us for an MOT today and after the 1 mile each way journey, it had a lot of steam coming from under the bonnet. When we opened the bonnet to investigate, we found the steam to be coming from the engine oil filler / breather not the radiator!
We pulled the dipstick and found lots of emulsion instead of oil, but the puzzling thing was the amount of coolant that had got into the oil in a short space of time – too much to be a head gasket and the car was still running sweetly.
I suspected a rusted through core plug in the head and upon removing the rocker cover I found coolant squirting out of 2 tiny holes in one of the core plugs. By the time I’d got the camera it was a welling, rather than squirting, but it did explain things. An easy repair by replacing the plug and a change of fluids later it was all was sorted. The other 2 core plugs were still nice and solid.

Core plug with holes

Video of coolant welling up from below!