Thanks for info Grumpy & Jaffa,
Have done a bit more cleaning before pulling it apart.

This one has a plastic fan on the flywheel that has been broken into many pieces.

Rocker cover & plate behind it were leaking, so this has contributed to all the cooling fins being totally blocked.

The valve has freed itself up and I can now feel good compression, smile

[Linked Image]

Happy is he who penetrates the mystery of things.