I’ve cleaned out the hole with a cleaning agent, removed the 18-14mm adaptor and filled the leaking hole with so-called Liquid Metal.
I found a suitable engineering-standard washer to fit under the adaptor and also covering the leaking hole to give the repair some mechanical strength, so tomorrow (my weekly run out) I will refill the rad with KSeal and top it up with water.
As long as it lasts a couple of months then good enough.
