A picture speaks a thousand words. Anyone care to bite?

Headgasket break

Similar breaks are repeated in a couple of other places, I'm guessing this is where my coolant is disappearing into the combustion chamber.

Underside of the head

I've already noticed some dirt on the under side of number 5's exhaust valve, the part that is cut and should seal with the seat, I doubt this can seal properly in its current condition. My fuel distributor was overhauled by Ken Mills Injection Limited a year or so ago. Perhaps cylinder 5 is this this sooty due to a leaking injector?

A typical cylinder

The visible strips look to be what Mercedes describe as friction marks due to frequent short journeys and is not a worry according to their literature.

As for the valve stem seals, I replaced them when I put the engine back together last summer. I've just checked its disc diameter, they are standard Mercedes valves. I will check the valve dimensions against the Merc specs. Anything else to check for on those? How about the seats, when do they need changing?