Leaking head from bolt

[Ashley Bremner]

I have an Ariel 350nh red hunter that is leaking oil out of the bolt that holds the head on by the spark plug,rear bolt. The oil is bubbling out of the bolt through the washer it appears. The engine has recently had a new head gasket but the leak is steadily getting worse. The bolts are well tightened after first couple of hundred miles of running in new piston. It would suggest that maybe the gasket wasn’t good or has blown. The barrel has always been a bit oily before the new piston and valve replacement. The surfaces of head and barrel were dead flat. Any other possibilities other than getting the head off? Ash