It looks like you've found the smoking gun, so to speak.
The plug looks like it has a burnt oily deposit and the piston crown appears to have a very heavy deposit also.
That indicates Flips excellent diagnosis would be correct. The problem might be gummed rings, or worst case scenario, broken rings! Or it could be the inlet guide seals.
Reddex might un-gum a stuck ring and also go some way to clearing the deposit on the piston (and the rest of the combustion chamber), but a broken ring would need a top end strip and a new ring(s), plus a de-coke while you're in there.
The valve seals are relatively easy to change, so long as you don't lose the valve down the cylinder. I'm not sure if they drop right through if the piston is at TDC, but I believe that filling the combustion chamber with string through the spark plug hole can save it from dropping. Another method is to fill the cylinder with air via an adapter in the plug hole... Obviously the rings need to be in good order for this to work. About 100psi is needed apparently.
Finally... As Flip says, use it more..
