to be fair, piston No 4 is clean, like it has been steamed!, just hope its sorted when i put the new gasket in. NEVER ever doing a head gasket on a Dispatch, Expert or a Skudo EVER again!!!!
If one of the pistons looks 'washed' then it does indicate that water/coolant has entered the cylinder. A clean piston crown when all the others are carbonised is classic sign of head gasket (or if fitted - wet liner problem)
A sweet smell from the exhaust is another sign and indicates that glycol in coolant is being drawn in to cylinder.
Sounds like the gasket is leaking from around coolant channels. As the piston comes up on compression stroke, air under pressure leaks back past gasket into cooling system thus pressurising the expansion tank. Some multi-layer or laminated head gaskets can leak between the layers when they 'delaminate' this is where coolant can flow under cylinder vacuum between layers and into cylinder.
Heads can crack between valve seats especially the exhaust valve seat where the water gallery is situated. They can also go porous which Vauxhalls were prone to not so many years ago. Cracks can also seal up when engine is cold and then expand when engine is to working temperature.
Let us know what they find.