oil leak - head gasket issue?

I am assuming you have recently rebuilt or redone the top end. After my rebuild I needed to re-torque the head four times in the first 600 miles, having new bolts and freshly annealed new copper gasket

I think the Dominator Service notes have the torque settings, but since I printed them off over two years ago, my recollection maybe wrong.

Search a bit on this website and you should come across the recommended settings and, as important, the tightening sequence.

You should not just torque up, but loosen slightly each bolt before re-torquing to ensure a true reading.

Generally it does pay to re-torque the cylinder head gasket after a recent rebuild. Alan is correct in suggesting that this might need to be done up to 4 times.
The choice of gasket material can be the cause of leaks.  Solid Copper gaskets need to be very carefully annealed or they will not seal properly. 
Similarly, a composite gasket will also allow the Black Gold to squeeze through any imperfections. 
Check out this diagram.

This shows what can happen if a composite head gasket is used on a Norton Twin engine. They have two weak points where crankcase pressure will push oil through the composite sandwich to the outer edge of the barrel/head joint.  The first being the push rod tunnels. Any splits in the gasket rings that seal the push rod tunnels will let oil straight past. 
The same can happen at the rear of the barrel where the oil return/drain hole passes through the gasket. For some reason, unless a solid copper gasket is used, there is no protection at all stopping oil from being pushed sideways inside the gasket in this area. It then heads for the nearest exit which happens to be the cylinder head bolt behind the spark plug.
I note in the photograph the head bolt behind the spark plug is wet with oil. Possibly agreeing with the text above.
 

Long reach bolts into cast iron blocks are entirely different to short threads in alloy.  

Some of the threads are likely to be somewhat compromised after all the years .i would use a small ring spanner about 6 inches long and just do up firmly and to the pattern. They will come loose in  50 miles anyway and need checking about 3 times till they settle . the 5/16 threads will need care and less force. I only ever strip threads when using a torque wrench !!.

Cylinder base nuts (3/8") 06.2652 25lbs/ft

Cylinder base nuts (5/16") 60.2321 20lbs/ft

Cylinder head nuts & bolts (3/8") various 30lbs/ft

Cylinder head nuts (5/16") 06.7529 20lbs/ft

All in the Dominator Service Notes which are available on this Web Site & free to NOC members.