Busy day today! Engine removed and stripped down. Looks all ok except bore #4 has a bit of pitting in the wall. Inspecting the head gasket around that area would suggest water has gotten into the bore at some point and it has sat.
The rear welsh plug was F**KED! Otherwise all appears to be good.
Hopefully boring out 40 thou over will clean up the pitting...if not, will have to get another block.