I believe someone mentioned crud in your holesBack up and running.
Gave everything a good polish and blew out with the air line. The flattened drill bit pulled some crud out of a few of the holes, in particular the middle bolt hole nearest where the gasket blew between 3 & 4.
Running very nicely atm, I've got some desperately needed materials to pick up from town tomorrow and when I've finished work I'll retorque and adjust tappets to be sure.