Oil leaking from head bolt

Since I had my 300tdi, there's been an oil leak from the top of the engine and down the passenger side of the front of the block. I've finally got it clean enough to see where it's coming from - under bolt number 7 in the image below. Does this indicate a head gasket on the way out, or are there any other potential causes to rule out first?

head gasket would be the most likely, unless oil is from rocker cover gasket

No, definitely around the bolt. It's slow enough that I can safely ignore it until there's a sensible opportunity to address it. I was meaning to change the timing belt too, so I guess I'll do them both at once - sometime after the MOT which is in October.