Update:
Replaced yesterday exhaust camshaft (with working decompressor part) and attempted to start but still unable to start the engine
Removed the starter motor and gave it to a friend who use rebuild stuff like this. He stripped it down, gave it a good clean up and refresh and fitted it this afternoon.
It's turning much better now (still lazy but that seems to be normal for these bikes). But after the fourth attempt over a couple of minutes it started. When the engine is warmed up and I turn it off it restarts every time on first or second attempt. I have left it a few hours and it started on the third attempt.
What I have also done is put back on the camshaft without the decompressor part fitted and tried this as a comparison. It still starts ok as well, though admittedly a little bit harder, you can tell the battery and starter struggles just that little bit more but it does start.
Hope this information helps someone.