I bought some 12.7 m12x1.75 bolts - my notes say 12@145mm and 5@172mm. I think I also bought some 150mm ones, so I'm not 100% sure which are on it.
They weren't sold as head bolts, just high tensile. I've had no problem with them.
As an update, it's still going well. It's developed a bit of an oil leak from the sump gasket, I think its because the sump is just touching the steering rack and the vibration may have worn through the gasket. I need to take the engine out in the next couple of months to confirm.