The culprit

After struggling to find any leak we we worried that the problem might be down to a failed main valley gasket. Fortunately it looks like this old rad is the reason. The freezing weather has just been too much for an old leaky and corroded radiator core. Topping it up with water just sees it run straight out. Fortunately a little warmer today, so managed to get it out.

Going shopping on the internet now. Maybe try a flashy new aluminium rally-style rad.