Head bolt leak.

[Bob Culver]

I suspect whatever modern substitute gaskets are made of, it is inferior to the old. I ran my Seven then a Javelin for decades with heads very frequently off both, and reused gaskets. The wet liner Jowett cars with different expansion rates are somewhat more prone to gasket trouble than other makes. But present owners seem to experience a lot of random head gasket failures at low mileage and this despite improved and standardised fitting procedures.