Thanks Harvey,
It's the original engine No 400-00233A so I'm keen to keep it alive if possible as it's so unique to the car. No idea if it was there before as i only spotted it when I cleaned it all up and just about to start the rebuild.
When I got the car it was clearly knackered and in need of a rebuild so I just removed it and left it alone until now......5 years later!
I was always minded to just refit and see what happens. It is right on the edge of the block and the only thing it compromises is the gearbox mount plate bolt but even then the crack is only on the top of the threaded tap.